LeetCode 387. First Unique Character in a String

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class Solution {
public:
    int firstUniqChar(string s) {
        unordered_map<int, int> frequency;
        for (char ch: s)
            ++frequency[ch];
        for (int i = 0; i < s.size(); ++i)
            if (frequency[s[i]] == 1)
                return i;
        return -1;
    }
};