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.

 1 bool isIsomorphic(char* s, char* t) {
 2     if(s == NULL || t == NULL)
 3        return false;
 4     int slen = strlen(s);
 5     int tlen = strlen(t);
 6     if(slen != tlen)
 7       return false;
 8     int index;
 9     int s_char_content[256];
10     int t_char_content[256];
11     for(index = 0; index < 256 ;index++)
12     {
13         s_char_content[index] = 257;
14         t_char_content[index] = 257;
15     }
16     
17     for(index = 0; index < slen; index++)
18     {
19         if(s_char_content[s[index]] == 257)
20         {
21             if(t_char_content[t[index]] == 257)
22             {
23                 s_char_content[s[index]] = t[index];
24                 t_char_content[t[index]] = s[index];
25             }
26             else return false;
27         }
28         else
29         {
30             if(s_char_content[s[index]] != t[index])
31                return false;
32         }
33     }
34     return true;
35 }