leetcode题解（三）： Longest Substring Without Repeating Characters

难度： Medium
题目描述：给定一个字符串，找出最长无重复子字符串（substring）

例子：
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.

思路： 滑动窗口（sliding window）
解法：滑动窗口+hashmap

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        int n = s.length(), ans = 0;
        Map<Character, Integer> map = new HashMap<>(); // current index of character
        // try to extend the range [i, j]
        //两个指针i和j
        for (int j = 0, i = 0; j < n; j++) {
        //如果s中包含现在j指向的字母的话
            if (map.containsKey(s.charAt(j))) {
            //更新 i 的位置 = j+1
                i = Math.max(map.get(s.charAt(j)), i);
            }
            //取最大的ans
            ans = Math.max(ans, j - i + 1);
            //在map中放入字母和index
            map.put(s.charAt(j), j + 1);
        }
        return ans;
    }
}