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.

分析：
我的想法是使用三个值front、back和mid分别指向子串的头尾和中间进行遍历，如下为遍历过程：

1. back每次加一
2. back加一后，mid对从front开始到back之前的元素进行遍历，并判断mid和back所在位置的元素值是否相同
   • 如果相同，front指向mid+1位置元素，然后break跳出mid的遍历
   • 如果不相同则mid++继续遍历直到相同或mid==back-1
3. 如果front到back之前的元素均与back不相同，则将back-front+1和现存最大的字串长度相比较，取大的赋给max。

代码：

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int max = 0;
        int len = s.size();
        if(len>0) max = 1;
        int front = 0;
        int back = front + 1;
        int mid = front;
        while(back<len){
            mid = front;
            bool flag = false;
            do{//用mid对从front到back之前的值进行遍历
                if(s[mid]==s[back]){//如果相同，字串头指向mid后一位
                    front = mid+1;
                    flag = true;
                    break;
                }else{//不相同继续遍历
                    mid++;
                }
            }while(mid<back);
            //如果front到back无重复元素，则将该子串长度与max比较，将最大的赋给max
            if(!flag) max = max<back-front+1?back-front+1:max;
            back++;
        }
        return max;
    }
};