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.

题目含义：获取最长子串，要去子串中没有重复字符

    public int lengthOfLongestSubstring(String s) {
        if (s.length() == 0) return 0;
        HashMap<Character,Integer> map = new HashMap<>(); //记录每个字符最后一次出现的位置
        int max=0;
        int j=0;
        for (int i=0;i<s.length();i++)
        {
            char letter = s.charAt(i);
            if (map.containsKey(letter))
            {
                j = Math.max(j,map.get(letter)+1); //j表示上一个letter后面的字符位置
            }
            map.put(letter,i);//记录每个字符最后一次出现的位置
            max = Math.max(max,i-j+1);//上一个letter后面的字符到i的长度，也就是非重复子串的长度
        }
        return max;        
    }