Find the Longest Substring Containing Vowels in Even Counts LeetCode solution in C++

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// When the xor of all the even times numbers are done it results in 0. The xor
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// of the vowels are done by indicating them with a single digit and the xor
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// value is stored in a map
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class Solution {
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 public:
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  int findTheLongestSubstring(string s) {
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    int x = 0;
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    unordered_map<int, int> mp;
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    mp[0] = -1;
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    int n = 0;
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    for (int i = 0; i < s.length(); i++) {
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      if (s[i] == 'a' || s[i] == 'e' || s[i] == 'i' || s[i] == 'o' || s[i] == 'u') {
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        x ^= (s[i] - 'a' + 1);
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        if (mp.find(x) == mp.end()) mp[x] = i;
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      }
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      if (mp.find(x) != mp.end()) n = max(n, i - mp[x]);
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    }
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    return n;
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  }
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};