leetCode 3. Longest Substring Without Repeating Characters

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

Example 1:

Input: "abcabcbb"
Output: 3 
Explanation: The answer is "abc", with the length of 3. 

Example 2:

Input: "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: "pwwkew"
Output: 3
Explanation: 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.

定义两个辅助数组：
　　last：在串0到（i-1）中，i位置字符最近出现的位置；
　　res：存放以i位置字符结尾的最长互不重复的子串长度；

class Solution8 {
    public int lengthOfLongestSubstring(String s) {
        if (s.length() < 1) return 0;
        if (s.length() == 1) return 1;
        int[] last = new int[s.length()];//存放在串0到(i-1)中，i位置字符最近出现的位置
        int[] res = new int[s.length()];//存放以i位置字符结尾的最长互不重复的子串长度
        int max = 1;
        Map<Character, Integer> map = new HashMap<>();
        for (int i = 0; i < s.length(); i++) {
            Integer lastLoc = map.get(s.charAt(i));
            if (lastLoc != null) {
                last[i] = lastLoc;
            }else {
                last[i] = -1;
            }
            map.put(s.charAt(i), i);
        }

        res[0] = 1;
        for (int i = 1; i < s.length(); i++) {
            if (last[i] < i - 1 - res[i-1]){
                res[i] = res[i-1] + 1;
            }else {
                res[i] = i - last[i];
            }
            max = (max < res[i]) ? res[i] : max;
        }
        return max;
    }
}