LeetCode-3-Longest Substring Without Repeating Characters

算法描述：

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.

解题思路：

这道题首先要区分 substring 和 subsequence 两个概念。substring 必须连续，而subsequence可以不连续。

substring相关的问题首先考虑滑动窗口机制，因此设置两个指针分别指向窗口左边和窗口右边。随着右指针的右移，窗口不断扩大。窗口更新分两种情况：当给定新字符在窗口中没有出现过，则将该字符加入窗口中；当该字符在窗口中出现过，则将窗口左指针更新为窗口中字符所在的位置。字符出现过的位置可以通过在窗口中搜索获得，但这样做显然浪费时间。因此可以用一个map来存放字符和其对应位置指针，算法中通过判断字符位置和左窗口指针的位置关系，以便快速更新。

int lengthOfLongestSubstring(string s){
    int res =0;
    int left = -1;
    unordered_map<char, int> unmp;  
    for(int i=0; i < s.size(); i++){
        if(unmp.find(s[i])!=unmp.end() && left < unmp[s[i]]){
            left = unmp[s[i]];
        }
        res = max(res, i - left);
        unmp[s[i]]=i;
    }    
    return res;
}