【leedcode】longest-substring-without-repeating-characters

Given a string, find the length of the longest substring without repeating characters.

Examples:

Given "abcabcbb", the answer is "abc", which the length is 3.

Given "bbbbb", the answer is "b", with the length of 1.

Given "pwwkew", the answer is "wke", with the length of 3. Note that the answer must be a substring, "pwke" is a subsequence and not a substring.

char[] buff;
    //used
    int used ;
    public int lastIndexOf(char ch, int endIndex) {
            int i = used;
            for (; i >= endIndex; i--) {
                if (buff[i] == ch) {
                    return i;
                }
            }
            return -1;
    }
    public int lastIndexOf(char ch) {
        return lastIndexOf(ch, 0);
    }
    
     public int lengthOfLongestSubstring(String s) {
             if (s == null || s.isEmpty()) {
                 return 0;
             }
            int len = s.length();
            buff = new  char[len];
            buff[0] = s.charAt(0);
            used = 1;
            char t;
            int idx = -1;
            int top = 0;
            int first = 0;
            while(used<len){
                t = s.charAt(used);
                idx = lastIndexOf(t, first);
//                System.err.println("["+top+"]s=" + new String(buff) + "," + (idx>-1 ? ( "pos=" +  idx + " find " + t ) : "" ) + ",idx="  + used + ",first=" + first);
                
                if (idx > -1) {
                    top = Math.max(used - first, top);
                    first = idx+1;
                }
                
                buff[used] = t;
                used++;
            } //            System.err.println("["+top+"]s=" + new String(buff) + "," + (idx>-1 ? ( "pos=" +  idx + " find " + t ) : "" ) + ",idx="  + used + ",first=" + first);
            return Math.max(len - first, top);
        }

 public int lengthOfLongestSubstring(String s) {
        int n = s.length(), ans = 0;
        int[] index = new int[128]; // current index of character
        // try to extend the range [i, j]
        for (int j = 0, i = 0; j < n; j++) {
            i = Math.max(index[s.charAt(j)], i);
            ans = Math.max(ans, j - i + 1);
            index[s.charAt(j)] = j + 1;
        }
        return ans;
    }