lintcode 中 tle 错误，求大佬帮忙看下

题目: Strings Homomorphism

public class Solution {
    /**
     * @param s a string
     * @param t a string
     * @return true if the characters in s can be replaced to get t or false
     */
    public boolean isIsomorphic(String s, String t) {
        // Write your code here
        if(s == null && t == null)  return true;
        else if(s == null || t == null) return false;
        else if(s.length() != t.length())   return false;
        else{
            String s1 = "";
            String s2 = "";
            int j = 0;
            for(int i = 0; i < s.length(); i++){
                int dis1 = s.indexOf(s.charAt(i));
                if(i == dis1){
                    s1 += j;
                    j++;
                }
                else{
                    s1 += s1.charAt(dis1);
                }
            }
            int l = 0;
            for(int k = 0; k < t.length(); k++){
                int dis2 = t.indexOf(t.charAt(k));
                if(k == dis2){
                    s2 += l;
                    l++;
                }
                else{
                    s2 += s2.charAt(dis2);
                }
            }
            boolean flag = s1.equals(s2);
            return flag;
        }
    }
}

还有另外一个 题目: First Position Unique Character

public class Solution {
    /*
     * @param s: a string
     * @return: it's index
     */
    public int firstUniqChar(String s) {
        // write your code here
        if (s == null) {
            return -1;
        }
        boolean flag = false;
        int i = 0;
        for (; i < s.length() - 1; i++) {
            String myStr = s.substring(0, i) + s.substring(i + 1);
            String ch = Character.toString(s.charAt(i));
            if (myStr.contains(ch)) {
                continue;
            }
            else {
                flag = true;
                break;
            }
        }
        if (flag) {
            return i;
        }
        else {
            return -1;
        }
    }
}