Leetcode_3. 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.
Seen this question in a real interview before?

思路：设m表示最近的没有重复的字符的下标位置。
令 L[i] = s[m…i]
那么当i+1个字符出现时，会有两种情况，若s[i+1]在s[m..i]中那么找s[i+1]在s[m…i]中出现的最近的位置m，那么
L[i+1] =s[m…i+1] ,若s[i+1没有出现在s[m…i]中，那么L[i+1] =s[m…i+1].字符的位置是否重复用map表示。
代码：

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        vector<int>mapIndex(256,-1);
        int m = 0,longest = 0 ;
        for(int i=0;i<s.length();i++)
        {
            m = max(mapIndex[s[i]]+1,m);
            mapIndex[s[i]] = i;
            longest = max(longest,i-m+1);
        }
        return longest;
    }
};