3. 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.

class Solution {
    public int lengthOfLongestSubstring(String s) {
        if(s == null){
            return 0;
        }
        char[] sc = s.toCharArray();
        if(sc.length > 1){
            return countLongestSubstring(sc);
        }
        return sc.length;
    }
    
    /**
     * counter longest substring 
     */
    private int countLongestSubstring(char[] sc){
        // current save substring list
        List<Character> currentSubstring = new ArrayList<>();
        // longest substring list
        List<Character> longestSubstring = new ArrayList<>();
        // flag index
        int flagIndex = 0;
        for (int i = 0;i < sc.length; i++) {
            char item = sc[i];
            if (!currentSubstring.contains(item)) {
                // add to list
                currentSubstring.add(item);
            } else {
                if (longestSubstring.size() < currentSubstring.size()) {
                    // evalution value
                    longestSubstring = currentSubstring;
                }
                // judge index
                if(flagIndex < sc.length -1){
                    i = flagIndex + 1;
                    flagIndex = i;
                    item = sc[i];
                }
                // init value
                currentSubstring = new ArrayList<>();
                currentSubstring.add(item);
            }
        }
        if (longestSubstring.size() < currentSubstring.size()) {
            // evalution value
            longestSubstring = currentSubstring;
        }
        // return result
        return longestSubstring.size();
    }
}