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.

1刷
Sliding Window，每次在HashMap里放入当前字符的index i。 要注意start何时更新。 比如abba这种情况。
Time Complexity - O(n)，Space Complexity - O(n)

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        int max = 0;
        if(s == null || s.length() == 0) return max;
        if(s.length()==1) return 1;
        int start = 0;
        //character to indices
        Map<Character, Integer> map = new HashMap<>();
        for(int i=0; i<s.length(); i++){
            Character c = s.charAt(i);
            if(map.containsKey(c)){
                if(map.get(c)>=start){
                    start = map.get(c) + 1;
                }
            }
            map.put(c, i);
            max = Math.max(max, i-start+1);
        }
        return max;
    }
}