leetcode:3. Longest Substring Without Repeating Characters (java)，leetcoderepeating

leetcode:3. Longest Substring Without Repeating Characters (java)，leetcoderepeating

题目链接：https://leetcode.com/problems/longest-substring-without-repeating-characters/

题目：

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.

解析：

利用hashmap存储不重复子串，key为字符，value为此字符的位置。从前向后进行遍历，只要map 中没有当前字符，便将其加入map 。并将子串长度加一。若当前字符已经出现在map 中，获得map中 此字符的位置，清除此位置以及之前的的所有key 。从此位置之后重新计算子串，保证了子串的不重复。

java 代码实现：

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        if(s == null) return 0;
        HashMap<Character, Integer> map = new HashMap<Character, Integer>();
        int start = 0;
        int maxLen = 0;
        int len = 0;
        for(int i = 0; i < s.length(); i++){
            if(!map.containsKey(s.charAt(i))){
                len++;
                if(len > maxLen) maxLen = len;
                map.put(s.charAt(i), i);
            }else{
                int index = map.get(s.charAt(i));
                for (int j = start; j <= index; j++){
                    map.remove(s.charAt(j));
                }
                map.put(s.charAt(i), i);
                start = index+1;
                len = i-index;
            }
        }
        return maxLen;
    }
}