[LeetCode] Isomorphic Strings

This problem can be solved by using a hash map to store the mapping between the two strings. I initially thought it only contained lowercase letters, but it actually includes all ASCII characters. I started with one hash map, but since the mapping needs to be one-to-one, I ended up using two.

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class Solution {
public:
    bool isIsomorphic(string s, string t) {
        if (s.length() != t.length()) return false;
        char hash_s[256];
        char hash_t[256];
        memset(hash_s, NULL, 256);
        memset(hash_t, NULL, 256);
        for (int i = 0; i < s.length(); i++) {
            if (NULL == hash_s[s[i]] && NULL == hash_t[t[i]]) {
                hash_s[s[i]] = t[i];
                hash_t[t[i]] = s[i];
            }
            else {
               if (hash_s[s[i]] != t[i] || hash_t[t[i]] != s[i]) return false;
            }
        }
        return true;
    }
};