【leetcode刷题笔记】Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters. For example, the longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.

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题解：贪心算法。

利用一个hashmap存放当前最长的无重substring所包含的元素，变量leftbound存放当前的最左边界。则当前遍历的字符s[i]有两种情况：

1. s[i]所指的元素已经存在在map中了，那么就左移leftbound直到leftbound到i这一段不再包含s[i]，并且在map中清除leftbound扫过的元素；
2. s[i]所指的元素不在map中，保持leftbound不动，查看是否需要更新当前最长substring的长度。

代码如下：

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        if(s == null || s.length() == 0)
            return 0;
            
        HashMap<Character, Integer> map = new HashMap<Character, Integer>();
        int leftebound = 0;
        int answer = 0;
       
        for(int i = 0;i < s.length();i++){
            char current = s.charAt(i);
            if(map.containsKey(s.charAt(i))){                
                while(s.charAt(leftebound) != current && leftebound < i){
                    map.remove(s.charAt(leftebound));
                    leftebound++;
                }
                leftebound++;
            }
            else {
                map.put(current, 0);
                answer = Math.max(i-leftebound+1, answer);
            }
        }
        
        return answer;    
    }
}