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.
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 :
- Count pairs with set bits sum equal to K
- Count pairs from two arrays having sum equal to K
- Check if count of Alphabets and count of Numbers are equal in the given String
- Count the pairs of vowels in the given string
- Count pairs of characters in a string whose ASCII value difference is K
- Count pairs of non-overlapping palindromic sub-strings of the given string
- Total character pairs from two strings, with equal number of set bits in their ascii value
- Count number of pairs (i, j) such that arr[i] * arr[j] = arr[i] + arr[j]
- String Range Queries to find the number of subsets equal to a given String
- Count pairs with Bitwise AND as ODD number
- Count number of triplets with product equal to given number with duplicates allowed
- Count number of triplets with product equal to given number with duplicates allowed | Set-2
- Count number of ordered pairs with Even and Odd Sums
- Count Substrings with equal number of 0s, 1s and 2s
- Count the number of subsequences of length k having equal LCM and HCF
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