Longest Substring- Deutsche Bank Online Assessment MNNIT Allahabad

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"Write a function: ```cpp int solution(string &S); ``` that, given a string `S` consisting of `N` lowercase English letters, returns the length of the longest substring in which every letter occurs an even number of times. A substring is defined as a contiguous segment of a string. If no such substring exists, return `0`. ### Examples **1.** Given `S = "bdaaadadb"`, the function should return `6`. Substrings in which every letter occurs an even number of times are `"aa"`, `"adad"`, `"daaada"` and `"aaadad"`. The length of the longest of them is `6`. **2.** Given `S = "abc"`, the function should return `0`. There is no non-empty substring in which every letter occurs an even number of times. **3.** Given `S = "zthzth"`, the function should return `6`. Every letter in the whole string occurs an even number of times. ### Write an efficient algorithm for the following assumptions: * `N` is an integer within the range `[1..100,000]`. * String `S` consists only of lowercase letters (`'a'..'z'`)."