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.

bool isIsomorphic(char* s, char* t) {
    if(s == NULL || t == NULL)
       return false;
    int slen = strlen(s);
    int tlen = strlen(t);
    if(slen != tlen)
      return false;
    int index;
    int s_char_content[256];
    int t_char_content[256];
    for(index = 0; index < 256 ;index++)
    {
        s_char_content[index] = 257;
        t_char_content[index] = 257;
    }
    
    for(index = 0; index < slen; index++)
    {
        if(s_char_content[s[index]] == 257)
        {
            if(t_char_content[t[index]] == 257)
            {
                s_char_content[s[index]] = t[index];
                t_char_content[t[index]] = s[index];
            }
            else return false;
        }
        else
        {
            if(s_char_content[s[index]] != t[index])
               return false;
        }
    }
    return true;
}