# K’th Non-repeating Character

• Difficulty Level : Medium
• Last Updated : 06 Jul, 2022

Given a string and a number k, find the k’th non-repeating character in the string. Consider a large input string with lacs of characters and a small character set. How to find the character by only doing only one traversal of input string?
Examples:

```Input : str = geeksforgeeks, k = 3
Output : r
First non-repeating character is f,
second is o and third is r.

Input : str = geeksforgeeks, k = 2
Output : o

Input : str = geeksforgeeks, k = 4
Output : Less than k non-repeating
characters in input.```

This problem is mainly an extension of First non-repeating character problem.
Method 1 (Simple : O(n2)
A Simple Solution is to run two loops. Start traversing from left side. For every character, check if it repeats or not. If the character doesn’t repeat, increment count of non-repeating characters. When the count becomes k, return the character.
Method 2 (O(n) but requires two traversals)

1. Create an empty hash.
2. Scan input string from left to right and insert values and their counts in the hash.
3. Scan input string from left to right and keep count of characters with counts more than 1. When count becomes k, return the character.

Method 3 (O(n) and requires one traversal)
The idea is to use two auxiliary arrays of size 256 (Assuming that characters are stored using 8 bits). The two arrays are:

```count[x] : Stores count of character 'x' in str.
If x is not present, then it stores 0.

index[x] : Stores indexes of non-repeating characters
in str. If a character 'x' is not  present
or x is repeating, then it stores  a value
that cannot be a valid index in str[]. For
example, length of string.```

1. Initialize all values in count[] as 0 and all values in index[] as n where n is length of string.
2. Traverse the input string str and do following for every character c = str[i].
• Increment count[x].
• If count[x] is 1, then store index of x in index[x], i.e., index[x] = i
• If count[x] is 2, then remove x from index[], i.e., index[x] = n
3. Now index[] has indexes of all non-repeating characters. Sort index[] in increasing order so that we get k’th smallest element at index[k]. Note that this step takes O(1) time because there are only 256 elements in index[].

Below is implementation of above idea.

## C++

 `// C++ program to find k'th non-repeating character` `// in a string` `#include ` `using` `namespace` `std;` `const` `int` `MAX_CHAR = 256;`   `// Returns index of k'th non-repeating character in` `// given string str[]` `int` `kthNonRepeating(string str, ``int` `k)` `{` `    ``int` `n = str.length();`   `    ``// count[x] is going to store count of` `    ``// character 'x' in str. If x is not present,` `    ``// then it is going to store 0.` `    ``int` `count[MAX_CHAR];`   `    ``// index[x] is going to store index of character` `    ``// 'x' in str.  If x is not  present or x is` `    ``// repeating, then it is going to store  a value` `    ``// (for example, length of string) that cannot be` `    ``// a valid index in str[]` `    ``int` `index[MAX_CHAR];`   `    ``// Initialize counts of all characters and indexes` `    ``// of non-repeating  characters.` `    ``for` `(``int` `i = 0; i < MAX_CHAR; i++)` `    ``{` `        ``count[i] = 0;` `        ``index[i] = n;  ``// A value more than any index` `                       ``// in str[]` `    ``}`   `    ``// Traverse the input string` `    ``for` `(``int` `i = 0; i < n; i++)` `    ``{` `        ``// Find current character and increment its` `        ``// count` `        ``char` `x = str[i];` `        ``++count[x];`   `        ``// If this is first occurrence, then set value` `        ``// in index as index of it.` `        ``if` `(count[x] == 1)` `            ``index[x] = i;`   `        ``// If character repeats, then remove it from` `        ``// index[]` `        ``if` `(count[x] == 2)` `            ``index[x] = n;` `    ``}`   `    ``// Sort index[] in increasing order.  This step` `    ``// takes O(1) time as size of index is 256 only` `    ``sort(index, index+MAX_CHAR);`   `    ``// After sorting, if index[k-1] is value, then ` `    ``// return it, else return -1.` `    ``return` `(index[k-1] != n)? index[k-1] : -1;` `}`   `// Driver code` `int` `main()` `{` `   ``string str = ``"geeksforgeeks"``;` `   ``int` `k = 3;` `   ``int` `res = kthNonRepeating(str, k);` `   ``(res == -1)? cout << ``"There are less than k non-"` `                        ``"repeating characters"` `              ``: cout << ``"k'th non-repeating character"` `                       ``" is  "``<< str[res];` `   ``return` `0;` `}`

## Java

 `// Java program to find k'th non-repeating character` `// in a string`   `import` `java.util.Arrays;`   `class` `GFG ` `{` `    ``public` `static` `int` `MAX_CHAR = ``256``;` `    `  `    ``// Returns index of k'th non-repeating character in` `    ``// given string str[]` `    ``static` `int` `kthNonRepeating(String str, ``int` `k)` `    ``{` `        ``int` `n = str.length();` ` `  `        ``// count[x] is going to store count of` `        ``// character 'x' in str. If x is not present,` `        ``// then it is going to store 0.` `        ``int``[] count = ``new` `int``[MAX_CHAR];` ` `  `        ``// index[x] is going to store index of character` `        ``// 'x' in str.  If x is not  present or x is` `        ``// repeating, then it is going to store  a value` `        ``// (for example, length of string) that cannot be` `        ``// a valid index in str[]` `        ``int``[] index = ``new` `int``[MAX_CHAR];` ` `  `        ``// Initialize counts of all characters and indexes` `        ``// of non-repeating  characters.` `        ``for` `(``int` `i = ``0``; i < MAX_CHAR; i++)` `        ``{` `            ``count[i] = ``0``;` `            ``index[i] = n;  ``// A value more than any index` `                           ``// in str[]` `        ``}` ` `  `        ``// Traverse the input string` `        ``for` `(``int` `i = ``0``; i < n; i++)` `        ``{` `            ``// Find current character and increment its` `            ``// count` `            ``char` `x = str.charAt(i);` `            ``++count[x];` ` `  `            ``// If this is first occurrence, then set value` `            ``// in index as index of it.` `            ``if` `(count[x] == ``1``)` `                ``index[x] = i;` ` `  `            ``// If character repeats, then remove it from` `            ``// index[]` `            ``if` `(count[x] == ``2``)` `                ``index[x] = n;` `        ``}` ` `  `        ``// Sort index[] in increasing order.  This step` `        ``// takes O(1) time as size of index is 256 only` `        ``Arrays.sort(index);` ` `  `        ``// After sorting, if index[k-1] is value, then ` `        ``// return it, else return -1.` `        ``return` `(index[k-``1``] != n)? index[k-``1``] : -``1``;` `    ``}` `    `  `    ``// driver program` `    ``public` `static` `void` `main (String[] args) ` `    ``{` `        ``String str = ``"geeksforgeeks"``;` `        ``int` `k = ``3``;` `        ``int` `res = kthNonRepeating(str, k);` `        `  `        ``System.out.println(res == -``1` `? ``"There are less than k non-repeating characters"` `:` `                           ``"k'th non-repeating character is  "` `+ str.charAt(res));` `    ``}` `}`   `// Contributed by Pramod Kumar`

## Python 3

 `# Python 3 program to find k'th ` `# non-repeating character in a string` `MAX_CHAR ``=` `256`   `# Returns index of k'th non-repeating ` `# character in given string str[]` `def` `kthNonRepeating(``str``, k):`   `    ``n ``=` `len``(``str``)`   `    ``# count[x] is going to store count of  ` `    ``# character 'x' in str. If x is not ` `    ``# present, then it is going to store 0.` `    ``count ``=` `[``0``] ``*` `MAX_CHAR`   `    ``# index[x] is going to store index of ` `    ``# character 'x' in str. If x is not ` `    ``# present or x is repeating, then it ` `    ``# is going to store a value (for example, ` `    ``# length of string) that cannot be a valid` `    ``# index in str[]` `    ``index ``=` `[``0``] ``*` `MAX_CHAR`   `    ``# Initialize counts of all characters ` `    ``# and indexes of non-repeating characters.` `    ``for` `i ``in` `range``( MAX_CHAR):` `        ``count[i] ``=` `0` `        ``index[i] ``=` `n ``# A value more than any ` `                     ``# index in str[]`   `    ``# Traverse the input string` `    ``for` `i ``in` `range``(n):` `        `  `        ``# Find current character and ` `        ``# increment its count` `        ``x ``=` `str``[i]` `        ``count[``ord``(x)] ``+``=` `1`   `        ``# If this is first occurrence, then ` `        ``# set value in index as index of it.` `        ``if` `(count[``ord``(x)] ``=``=` `1``):` `            ``index[``ord``(x)] ``=` `i`   `        ``# If character repeats, then remove ` `        ``# it from index[]` `        ``if` `(count[``ord``(x)] ``=``=` `2``):` `            ``index[``ord``(x)] ``=` `n`   `    ``# Sort index[] in increasing order. This step` `    ``# takes O(1) time as size of index is 256 only` `    ``index.sort()`   `    ``# After sorting, if index[k-1] is value, ` `    ``# then return it, else return -1.` `    ``return` `index[k ``-` `1``] ``if` `(index[k ``-` `1``] !``=` `n) ``else` `-``1`   `# Driver code` `if` `__name__ ``=``=` `"__main__"``:` `    ``str` `=` `"geeksforgeeks"` `    ``k ``=` `3` `    ``res ``=` `kthNonRepeating(``str``, k)` `    ``if``(res ``=``=` `-``1``):` `        ``print``(``"There are less than k"``,` `              ``"non-repeating characters"``)` `    ``else``:` `        ``print``(``"k'th non-repeating character is"``, ` `                                       ``str``[res])`   `# This code is contributed` `# by ChitraNayal`

## C#

 `// C# program to find k'th non-repeating` `// character in a string` `using` `System;`   `class` `GFG {` `    `  `    ``public` `static` `int` `MAX_CHAR = 256;` `    `  `    ``// Returns index of k'th non-repeating ` `    ``// character in given string str[]` `    ``static` `int` `kthNonRepeating(String str, ``int` `k)` `    ``{` `        `  `        ``int` `n = str.Length;`   `        ``// count[x] is going to store count of` `        ``// character 'x' in str. If x is not` `        ``// present, then it is going to store 0.` `        ``int` `[]count = ``new` `int``[MAX_CHAR];`   `        ``// index[x] is going to store index of` `        ``// character 'x' in str. If x is not` `        ``// present or x is repeating, then it` `        ``// is going to store a value (for ` `        ``// example, length of string) that` `        ``// cannot be a valid index in str[]` `        ``int` `[]index = ``new` `int``[MAX_CHAR];`   `        ``// Initialize counts of all characters` `        ``// and indexes of non-repeating ` `        ``// characters.` `        ``for` `(``int` `i = 0; i < MAX_CHAR; i++)` `        ``{` `            ``count[i] = 0;` `            `  `            ``// A value more than any index` `            ``// in str[]` `            ``index[i] = n; ` `        ``}`   `        ``// Traverse the input string` `        ``for` `(``int` `i = 0; i < n; i++)` `        ``{` `            `  `            ``// Find current character and ` `            ``// increment its count` `            ``char` `x = str[i];` `            ``++count[x];`   `            ``// If this is first occurrence,` `            ``// then set value in index as` `            ``// index of it.` `            ``if` `(count[x] == 1)` `                ``index[x] = i;`   `            ``// If character repeats, then ` `            ``// remove it from index[]` `            ``if` `(count[x] == 2)` `                ``index[x] = n;` `        ``}`   `        ``// Sort index[] in increasing order. ` `        ``// This step takes O(1) time as size` `        ``// of index is 256 only` `        ``Array.Sort(index);`   `        ``// After sorting, if index[k-1] is` `        ``// value, then return it, else ` `        ``// return -1.` `        ``return` `(index[k-1] != n) ?` `                           ``index[k-1] : -1;` `    ``}` `    `  `    ``// driver program` `    ``public` `static` `void` `Main () ` `    ``{` `        ``String str = ``"geeksforgeeks"``;` `        ``int` `k = 3;` `        ``int` `res = kthNonRepeating(str, k);` `        `  `    ``Console.Write(res == -1 ? ``"There are less"` `         ``+ ``" than k non-repeating characters"` `:` `            ``"k'th non-repeating character is "` `                                   ``+ str[res]);` `    ``}` `}`   `// This code is contributed by nitin mittal.`

## Javascript

 ``

Output:

`k'th non-repeating character is  r`

Time Complexity: O(m log m) , here m is the MAX_CHARS = 256
Auxiliary Space: O(m), here m is the MAX_CHARS = 256

Space Optimized Solution :
This can be space optimized and can be solved using single index array only. Below is the space optimized solution:

## C++

 `#include ` `using` `namespace` `std;` `#define MAX_CHAR 256` `int` `kthNonRepeating(string input, ``int` `k)` `{` `    ``int` `inputLength = input.length();`   `    ``/*` `     ``* indexArr will store index of non-repeating` `     ``* characters, inputLength for characters not in input` `     ``* and inputLength+1 for repeated characters.` `     ``*/` `    ``int` `indexArr[MAX_CHAR];`   `    ``// initialize all values in indexArr as inputLength.` `    ``for` `(``int` `i = 0; i < MAX_CHAR; i++) {` `        ``indexArr[i] = inputLength;` `    ``}` `    ``for` `(``int` `i = 0; i < inputLength; i++) {` `        ``char` `c = input[i];` `        ``if` `(indexArr == inputLength) {` `            ``indexArr = i;` `        ``}` `        ``else` `{` `            ``indexArr = inputLength + 2;` `        ``}` `    ``}`   `    ``sort(indexArr, indexArr + MAX_CHAR);` `    ``return` `(indexArr[k - 1] != inputLength)` `               ``? indexArr[k - 1]` `               ``: -1;` `}`   `int` `main()` `{` `    ``string input = ``"geeksforgeeks"``;` `    ``int` `k = 3;` `    ``int` `res = kthNonRepeating(input, k);` `    ``if` `(res == -1)` `        ``cout << ``"There are less than k non-repeating "` `                ``"characters"``;` `    ``else` `        ``cout << ``"k'th non-repeating character is  "` `             ``<< input[res];` `    ``return` `0;` `}`   `// This code is contributed by gauravrajput1`

## Java

 `import` `java.util.*;`   `public` `class` `GFG {` `    ``public` `static` `int` `MAX_CHAR = ``256``; ` `    `  `    ``public` `static` `void` `main (String[] args)  ` `    ``{ ` `        ``final` `String input = ``"geeksforgeeks"``; ` `        ``int` `k = ``3``; ` `        ``int` `res = kthNonRepeating(input, k); ` `          `  `        ``System.out.println(res == -``1` `? ``"There are less than k non-repeating characters"` `: ` `                           ``"k'th non-repeating character is  "` `+ input.charAt(res)); ` `    ``}`   `    ``public` `static` `int` `kthNonRepeating(``final` `String input, ``final` `int` `k) {` `        ``final` `int` `inputLength = input.length();`   `        ``/*` `         ``* indexArr will store index of non-repeating characters,` `         ``* inputLength for characters not in input and` `         ``* inputLength+1 for repeated characters.` `         ``*/` `        ``final` `int``[] indexArr = ``new` `int``[MAX_CHAR];` `        `  `        ``// initialize all values in indexArr as inputLength.` `        ``Arrays.fill(indexArr, inputLength);` `        `  `        ``for` `(``int` `i = ``0``; i < inputLength ; i++) {` `            ``final` `char` `c = input.charAt(i);` `            ``if` `(indexArr == inputLength) {` `                ``indexArr = i;` `            ``} ``else` `{` `                ``indexArr = inputLength + ``2``;` `            ``}` `        ``}` `        `  `        ``Arrays.sort(indexArr);` `        `  `        ``return` `(indexArr[k-``1``] != inputLength) ? indexArr[k-``1``] : -``1``; ` `    ``}` `}` `// Contributed by AK`

## Python3

 `MAX_CHAR ``=` `256`   `def` `kthNonRepeating(``Input``,k):` `    ``inputLength ``=` `len``(``Input``)` `    `  `    ``# indexArr will store index of non-repeating characters,` `    ``# inputLength for characters not in input and` `    ``# inputLength+1 for repeated characters.` `    `  `    ``# initialize all values in indexArr as inputLength.` `    ``indexArr ``=` `[inputLength ``for` `i ``in` `range``(MAX_CHAR)]` `    `  `    ``for` `i ``in` `range``(inputLength):` `        ``c ``=` `Input``[i]` `        ``if` `(indexArr[``ord``(c)] ``=``=` `inputLength):` `            ``indexArr[``ord``(c)] ``=` `i` `        ``else``:` `            ``indexArr[``ord``(c)] ``=` `inputLength ``+` `2` `    ``indexArr.sort()` `    ``if``(indexArr[k ``-` `1``] !``=` `inputLength):` `        ``return` `indexArr[k ``-` `1``]` `    ``else``:` `        ``return` `-``1`   `Input` `=` `"geeksforgeeks"` `k ``=` `3` `res ``=` `kthNonRepeating(``Input``, k)` `if``(res ``=``=` `-``1``):` `    ``print``(``"There are less than k non-repeating characters"``)` `else``:` `    ``print``(``"k'th non-repeating character is"``, ``Input``[res])` `    `  `# This code is contributed by rag2127`

## C#

 `using` `System;`   `public` `class` `GFG` `{` `    ``public` `static` `int` `MAX_CHAR = 256;` `    ``static` `public` `void` `Main ()` `    ``{` `        ``string` `input = ``"geeksforgeeks"``;  ` `        ``int` `k = 3;  ` `        ``int` `res = kthNonRepeating(input, k);  ` `            `  `        ``Console.WriteLine(res == -1 ? ``"There are less than k non-repeating characters"` `:  ` `                           ``"k'th non-repeating character is  "` `+ input[res]);` `    ``}` `    `  `    ``public` `static` `int` `kthNonRepeating(``string` `input, ``int` `k) { ` `        ``int` `inputLength = input.Length; ` `  `  `        ``/* ` `         ``* indexArr will store index of non-repeating characters, ` `         ``* inputLength for characters not in input and ` `         ``* inputLength+1 for repeated characters. ` `         ``*/` `        ``int``[] indexArr = ``new` `int``[MAX_CHAR]; ` `          `  `        ``// initialize all values in indexArr as inputLength. ` `        ``Array.Fill(indexArr, inputLength); ` `          `  `        ``for` `(``int` `i = 0; i < inputLength ; i++) { ` `            ``char` `c = input[i]; ` `            ``if` `(indexArr == inputLength) { ` `                ``indexArr = i; ` `            ``} ``else` `{ ` `                ``indexArr = inputLength + 2; ` `            ``} ` `        ``} ` `          `  `        ``Array.Sort(indexArr); ` `        ``return` `(indexArr[k - 1] != inputLength) ? indexArr[k - 1] : -1;  ` `    ``}` `}`   `// This code is contributed by avanitrachhadiya2155`

## Javascript

 ``

Output:

`k'th non-repeating character is  r`

Time Complexity: O(m log m) , here m is the MAX_CHARS = 256
Auxiliary Space: O(m), here m is the MAX_CHARS = 256

This article is contributed by Aarti_Rathi and Shivam Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.