Leecode#3 Longest Substring Without Repeating Characters

一、 问题描述
Leecode第三题，题目为：
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.

先把问题翻译一下：
给定一个字符串，在字符不重复的情况下返回最长子字符串的长度。
二、解题思路
（1）从字符串第一个字符至最后一个字符顺序遍历字符串，并标记该不重复字符串的长度，for example：len=k,(k为不重复字符串的长度)；
（2）继续遍历字符串，如果有不重复字符串length大于已标记字符串长度，那么更新符号变量的值，否则，继续遍历，直至最后一个字符串。
（3）返回最长字符串的内容及长度。

三、实现代码

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
          string max_string = ""; 
          string goal = ""; 
          char ch; 
          for(int i=0; i<s.length(); i++)
          {   
              ch = s[i];
              int idx = goal.find(ch);
              if(idx==string::npos)
              {   
                  goal += s[i];
              }   
              else
              {   
                  goal = goal.substr(idx+1);
                  goal += s[i];
              }   
              if(goal.length()>=max_string.length())
              {   
                 max_string = goal;
              }   
          }   
          return max_string.length();
    }
};