LeetCode 389. Find the Difference

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class Solution {
public:
    char findTheDifference(string s, string t) {
        int bits = 0;
        for (char c : s)
            bits ^= (c - 'a');
        for (char c : t)
            bits ^= (c - 'a');
        return bits + 'a';
    }
};
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class Solution {
public:
    char findTheDifference(string s, string t) {
        unordered_map<char, int> m;
        for (char c : s)
            ++m[c];
        for (char c : t)
            if (--m[c] < 0)
                return c;
        return ' ';
    }
};
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class Solution {
public:
    char findTheDifference(string s, string t) {
        int as = 0, at = 0;
        for (char ch: s)
            as += (ch - 'a');
        for (char ch: t)
            at += (ch - 'a');
        return at - as + 'a';
    }
};