LeetCode Isomorphic Strings

Given two strings s and t, determine if they are isomorphic.

Two strings are isomorphic if the characters in s can be replaced to get t.

All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character but a character may map to itself.

For example,
Given "egg", "add", return true.

Given "foo", "bar", return false.

Given "paper", "title", return true.

Note:
You may assume both s and t have the same length.

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// 08:18
class Solution {
public:
    bool isIsomorphic(string s, string t) {
        int slen = s.size();
        int tlen = t.size();
        if (slen != tlen) {
            return false;
        }
        char tbl[256] = {0};
        bool used[256] = {0};
        for (int i=0; i<slen; i++) {
            char ch = s[i];
            if (tbl[ch] == 0 && !used[t[i]]) {
                tbl[ch] = t[i];
                used[t[i]] = true;
                continue;
            }
            if (tbl[ch] != t[i]) {
                return false;
            }
        }
        return true;
    }
};