# Count number of equal pairs in a string

Given a string s, find the number of pairs of characters that are same. Pairs (s[i], s[j]), (s[j], s[i]), (s[i], s[i]), (s[j], s[j]) should be considered different.

**Examples :**

Input: air Output: 3 Explanation : 3 pairs that are equal are (a, a), (i, i) and (r, r) Input : geeksforgeeks Output : 31

The **naive approach** will be you to run two nested for loops and find out all pairs and keep a count of all pairs. But this is not efficient enough for longer length of strings.

For an **efficient approach**, we need to count the number of equal pairs in **linear time**. Since pairs (x, y) and pairs (y, x) are considered different. We need to use a hash table to store the count of all occurrences of a character.So we know if a character occurs twice, then it will have 4 pairs – **(i, i), (j, j), (i, j), (j, i)**. So using a hash function, store the occurrence of each character, then for each character the number of pairs will be occurrence^2. Hash table will be 256 in length as we have 256 characters.

Below is the implementation of the above approach :

## C++

`// CPP program to count the number of pairs ` `#include <bits/stdc++.h> ` `using` `namespace` `std; ` `#define MAX 256 ` ` ` `// Function to count the number of equal pairs ` `int` `countPairs(string s) ` `{ ` ` ` `// Hash table ` ` ` `int` `cnt[MAX] = { 0 }; ` ` ` ` ` `// Traverse the string and count occurrence ` ` ` `for` `(` `int` `i = 0; i < s.length(); i++) ` ` ` `cnt[s[i]]++; ` ` ` ` ` `// Stores the answer ` ` ` `int` `ans = 0; ` ` ` ` ` `// Traverse and check the occurrence of every character ` ` ` `for` `(` `int` `i = 0; i < MAX; i++) ` ` ` `ans += cnt[i] * cnt[i]; ` ` ` ` ` `return` `ans; ` `} ` ` ` `// Driver Code ` `int` `main() ` `{ ` ` ` `string s = ` `"geeksforgeeks"` `; ` ` ` `cout << countPairs(s); ` ` ` `return` `0; ` `} ` |

## Java

`// Java program to count the number of pairs ` `import` `java.io.*; ` ` ` `class` `GFG { ` ` ` ` ` `static` `int` `MAX = ` `256` `; ` ` ` ` ` `// Function to count the number of equal pairs ` ` ` `static` `int` `countPairs(String s) ` ` ` `{ ` ` ` `// Hash table ` ` ` `int` `cnt[] = ` `new` `int` `[MAX]; ` ` ` ` ` `// Traverse the string and count occurrence ` ` ` `for` `(` `int` `i = ` `0` `; i < s.length(); i++) ` ` ` `cnt[s.charAt(i)]++; ` ` ` ` ` `// Stores the answer ` ` ` `int` `ans = ` `0` `; ` ` ` ` ` `// Traverse and check the occurrence ` ` ` `// of every character ` ` ` `for` `(` `int` `i = ` `0` `; i < MAX; i++) ` ` ` `ans += cnt[i] * cnt[i]; ` ` ` ` ` `return` `ans; ` ` ` `} ` ` ` ` ` `// Driver Code ` ` ` `public` `static` `void` `main (String[] args) ` ` ` `{ ` ` ` `String s = ` `"geeksforgeeks"` `; ` ` ` `System.out.println(countPairs(s)); ` ` ` `} ` `} ` ` ` `// This code is contributed by vt_m ` |

## Python 3

`# Python3 program to count the ` `# number of pairs ` `MAX` `=` `256` ` ` `# Function to count the number ` `# of equal pairs ` `def` `countPairs(s): ` ` ` ` ` `# Hash table ` ` ` `cnt ` `=` `[` `0` `for` `i ` `in` `range` `(` `0` `, ` `MAX` `)] ` ` ` ` ` `# Traverse the string and count ` ` ` `# occurrence ` ` ` `for` `i ` `in` `range` `(` `len` `(s)): ` ` ` `cnt[` `ord` `(s[i]) ` `-` `97` `] ` `+` `=` `1` ` ` ` ` `# Stores the answer ` ` ` `ans ` `=` `0` ` ` ` ` `# Traverse and check the occurrence ` ` ` `# of every character ` ` ` `for` `i ` `in` `range` `(` `0` `, ` `MAX` `): ` ` ` `ans ` `+` `=` `cnt[i] ` `*` `cnt[i] ` ` ` ` ` `return` `ans ` ` ` `# Driver code ` `if` `__name__` `=` `=` `"__main__"` `: ` ` ` `s ` `=` `"geeksforgeeks"` ` ` `print` `(countPairs(s)) ` ` ` `# This code is contributed ` `# by Sairahul099 ` |

## C#

`// C# program to count the number of pairs ` `using` `System; ` ` ` `class` `GFG { ` ` ` ` ` `static` `int` `MAX = 256; ` ` ` ` ` `// Function to count the number of equal pairs ` ` ` `static` `int` `countPairs(` `string` `s) ` ` ` `{ ` ` ` `// Hash table ` ` ` `int` `[]cnt = ` `new` `int` `[MAX]; ` ` ` ` ` `// Traverse the string and count occurrence ` ` ` `for` `(` `int` `i = 0; i < s.Length; i++) ` ` ` `cnt[s[i]]++; ` ` ` ` ` `// Stores the answer ` ` ` `int` `ans = 0; ` ` ` ` ` `// Traverse and check the occurrence ` ` ` `// of every character ` ` ` `for` `(` `int` `i = 0; i < MAX; i++) ` ` ` `ans += cnt[i] * cnt[i]; ` ` ` ` ` `return` `ans; ` ` ` `} ` ` ` ` ` `// Driver Code ` ` ` `public` `static` `void` `Main () ` ` ` `{ ` ` ` `string` `s = ` `"geeksforgeeks"` `; ` ` ` `Console.WriteLine(countPairs(s)); ` ` ` `} ` `} ` ` ` `// This code is contributed by vt_m ` |

**Output :**

31