# Swap all occurrences of two characters to get lexicographically smallest string

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

Given string str of lower case English alphabets. One can choose any two characters in the string and replace all the occurrences of the first character with the second character and replace all the occurrences of the second character with the first character. Find the lexicographically smallest string that can be obtained by doing this operation at most once. Examples:

Output: aacd
Swap all the occurrences of ‘c’ with ‘a’ and all the occurrences of ‘a’ with ‘c’ to get “aacd” which is the lexicographically smallest string that we can get.

Input: str = “abba”
Output: abba
The only possible operation will convert the given string to “baab” which is not lexicographically smallest.

Approach:

• First, we store the first appearance of every character in a string in a hash array chk[].
• In order to find the lexicographically smaller string, the leftmost character must be replaced with some character which is smaller than it. This will only happen if the smaller character appears after it in the array.
• So, start traversing the string from the left and for every character, find the smallest character (even smaller than the current character) that appears after swapping all of their occurrences to get the required string.
• If no such character pair is found in the previous string then print the given string as it is the smallest string possible.

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach` `#include ` `using` `namespace` `std;` `  `  `#define MAX 26` `  `  `// Function to return the lexicographically` `// smallest string after swapping all the` `// occurrences of any two characters` `string smallestStr(string str, ``int` `n)` `{` `    ``int` `i, j;` `    ``// To store the first index of` `    ``// every character of str` `    ``int` `chk[MAX];` `    ``for` `(i = 0; i < MAX; i++)` `        ``chk[i] = -1;` `  `  `    ``// Store the first occurring` `    ``// index every character` `    ``for` `(i = 0; i < n; i++) {` `  `  `        ``// If current character is appearing` `        ``// for the first time in str` `        ``if` `(chk[str[i] - ``'a'``] == -1)` `            ``chk[str[i] - ``'a'``] = i;` `    ``}` `  `  `    ``// Starting from the leftmost character` `    ``for` `(i = 0; i < n; i++) {` `  `  `        ``bool` `flag = ``false``;` `  `  `        ``// For every character smaller than str[i]` `        ``for` `(j = 0; j < str[i] - ``'a'``; j++) {` `  `  `            ``// If there is a character in str which is` `            ``// smaller than str[i] and appears after it` `            ``if` `(chk[j] > chk[str[i] - ``'a'``]) {` `                ``flag = ``true``;` `                ``break``;` `            ``}` `        ``}` `  `  `        ``// If the required character pair is found` `        ``if` `(flag)` `            ``break``;` `    ``}` `  `  `    ``// If swapping is possible` `    ``if` `(i < n-1) {` `  `  `        ``// Characters to be swapped` `        ``char` `ch1 = str[i];` `        ``char` `ch2 = ``char``(j + ``'a'``);` `  `  `        ``// For every character` `        ``for` `(i = 0; i < n; i++) {` `  `  `            ``// Replace every ch1 with ch2` `            ``// and every ch2 with ch1` `            ``if` `(str[i] == ch1)` `                ``str[i] = ch2;` `  `  `            ``else` `if` `(str[i] == ch2)` `                ``str[i] = ch1;` `        ``}` `    ``}` `  `  `    ``return` `str;` `}` `  `  `// Driver code` `int` `main()` `{` `    ``string str = ``"ccad"``;` `    ``int` `n = str.length();` `  `  `    ``cout << smallestStr(str, n);` `  `  `    ``return` `0;` `}`

## Java

 `// Java implementation of the approach ` `import` `java.util.*;` `  `  `class` `GFG` `{` `static` `int` `MAX = ``26``;` `  `  `// Function to return the lexicographically` `// smallest string after swapping all the` `// occurrences of any two characters` `static` `String smallestStr(``char` `[]str, ``int` `n)` `{` `    ``int` `i, j = ``0``;` `      `  `    ``// To store the first index of` `    ``// every character of str` `    ``int` `[]chk = ``new` `int``[MAX];` `    ``for` `(i = ``0``; i < MAX; i++)` `        ``chk[i] = -``1``;` `  `  `    ``// Store the first occurring` `    ``// index every character` `    ``for` `(i = ``0``; i < n; i++) ` `    ``{` `  `  `        ``// If current character is appearing` `        ``// for the first time in str` `        ``if` `(chk[str[i] - ``'a'``] == -``1``)` `            ``chk[str[i] - ``'a'``] = i;` `    ``}` `  `  `    ``// Starting from the leftmost character` `    ``for` `(i = ``0``; i < n; i++)` `    ``{` `        ``boolean` `flag = ``false``;` `  `  `        ``// For every character smaller than str[i]` `        ``for` `(j = ``0``; j < str[i] - ``'a'``; j++) ` `        ``{` `  `  `            ``// If there is a character in str which is` `            ``// smaller than str[i] and appears after it` `            ``if` `(chk[j] > chk[str[i] - ``'a'``]) ` `            ``{` `                ``flag = ``true``;` `                ``break``;` `            ``}` `        ``}` `  `  `        ``// If the required character pair is found` `        ``if` `(flag)` `            ``break``;` `    ``}` `  `  `    ``// If swapping is possible` `    ``if` `(i < n-``1``) ` `    ``{` `  `  `        ``// Characters to be swapped` `        ``char` `ch1 = str[i];` `        ``char` `ch2 = (``char``) (j + ``'a'``);` `  `  `        ``// For every character` `        ``for` `(i = ``0``; i < n; i++)` `        ``{` `  `  `            ``// Replace every ch1 with ch2` `            ``// and every ch2 with ch1` `            ``if` `(str[i] == ch1)` `                ``str[i] = ch2;` `  `  `            ``else` `if` `(str[i] == ch2)` `                ``str[i] = ch1;` `        ``}` `    ``}` `  `  `    ``return` `String.valueOf(str);` `}` `  `  `// Driver code` `public` `static` `void` `main(String[] args)` `{` `    ``String str = ``"ccad"``;` `    ``int` `n = str.length();` `  `  `    ``System.out.println(smallestStr(` `                       ``str.toCharArray(), n));` `}` `}`

## Python

 `# python3 implementation of the approach` `MAX``=``256` `  `  `# Function to return the lexicographically` `# smallest after swapping all the` `# occurrences of any two characters` `def` `smallestStr(``str``, n):` `    ``i, j``=``0``,``0` `    ``# To store the first index of` `    ``# every character of str` `    ``chk``=``[``0` `for` `i ``in` `range``(``MAX``)]` `    ``for` `i ``in` `range``(``MAX``):` `        ``chk[i] ``=` `-``1` `  `  `    ``# Store the first occurring` `    ``# index every character` `    ``for` `i ``in` `range``(n):` `  `  `        ``# If current character is appearing` `        ``# for the first time in str` `        ``if` `(chk[``ord``(``str``[i])] ``=``=` `-``1``):` `            ``chk[``ord``(``str``[i])] ``=` `i` `  `  `    ``# Starting from the leftmost character` `    ``for`  `i ``in` `range``(n):` `        ``flag ``=` `False` `  `  `        ``# For every character smaller than ord(str[i])` `        ``for` `j ``in` `range``(``ord``(``str``[i])):` `  `  `            ``# If there is a character in str which is` `            ``# smaller than ord(str[i]) and appears after it` `            ``if` `(chk[j] > chk[``ord``(``str``[i])]):` `                ``flag ``=` `True` `                ``break` `  `  `  `  `        ``# If the required character pair is found` `        ``if` `(flag):` `            ``break` `  `  `    ``# If swapping is possible` `    ``if` `(i < n``-``1``):` `  `  `        ``# Characters to be swapped` `        ``ch1 ``=` `(``str``[i])` `        ``ch2 ``=` `chr``(j)` `  `  `        ``# For every character` `        ``for` `i ``in` `range``(n):` `  `  `            ``# Replace every ch1 with ch2` `            ``# and every ch2 with ch1` `            ``if` `(``str``[i] ``=``=` `ch1):` `                ``str``[i] ``=` `ch2` `  `  `            ``elif` `(``str``[i] ``=``=` `ch2):` `                ``str``[i] ``=` `ch1` `  `  `    ``return` `"".join(``str``)` `  `  `  `  `# Driver code` `  `  `st ``=` `"ccad"` `str``=``[i ``for` `i ``in` `st]` `n ``=` `len``(``str``)` `  `  `print``(smallestStr(``str``, n))`

## C#

 `// C# implementation of the approach` `using` `System;` `      `  `class` `GFG` `{` `static` `int` `MAX = 26;` `  `  `// Function to return the lexicographically` `// smallest string after swapping all the` `// occurrences of any two characters` `static` `String smallestStr(``char` `[]str, ``int` `n)` `{` `    ``int` `i, j = 0;` `      `  `    ``// To store the first index of` `    ``// every character of str` `    ``int` `[]chk = ``new` `int``[MAX];` `    ``for` `(i = 0; i < MAX; i++)` `        ``chk[i] = -1;` `  `  `    ``// Store the first occurring` `    ``// index every character` `    ``for` `(i = 0; i < n; i++) ` `    ``{` `  `  `        ``// If current character is appearing` `        ``// for the first time in str` `        ``if` `(chk[str[i] - ``'a'``] == -1)` `            ``chk[str[i] - ``'a'``] = i;` `    ``}` `  `  `    ``// Starting from the leftmost character` `    ``for` `(i = 0; i < n; i++)` `    ``{` `        ``Boolean flag = ``false``;` `  `  `        ``// For every character smaller than str[i]` `        ``for` `(j = 0; j < str[i] - ``'a'``; j++) ` `        ``{` `  `  `            ``// If there is a character in str which is` `            ``// smaller than str[i] and appears after it` `            ``if` `(chk[j] > chk[str[i] - ``'a'``]) ` `            ``{` `                ``flag = ``true``;` `                ``break``;` `            ``}` `        ``}` `  `  `        ``// If the required character pair is found` `        ``if` `(flag)` `            ``break``;` `    ``}` `  `  `    ``// If swapping is possible` `    ``if` `(i < n-1) ` `    ``{` `  `  `        ``// Characters to be swapped` `        ``char` `ch1 = str[i];` `        ``char` `ch2 = (``char``) (j + ``'a'``);` `  `  `        ``// For every character` `        ``for` `(i = 0; i < n; i++)` `        ``{` `  `  `            ``// Replace every ch1 with ch2` `            ``// and every ch2 with ch1` `            ``if` `(str[i] == ch1)` `                ``str[i] = ch2;` `  `  `            ``else` `if` `(str[i] == ch2)` `                ``str[i] = ch1;` `        ``}` `    ``}` `  `  `    ``return` `String.Join(``""``, str);` `}` `  `  `// Driver code` `public` `static` `void` `Main(String[] args)` `{` `    ``String str = ``"ccad"``;` `    ``int` `n = str.Length;` `  `  `    ``Console.WriteLine(smallestStr(` `                      ``str.ToCharArray(), n));` `}` `}`

## Javascript

 ``

Output

`aacd`

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