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.

题目大意：

  给定一个字符串，找到无重复字符的最长子串的长度。
  提示：是找子串而不是子序列。

解题思路：

  第一，要找到最长，肯定得把字符串遍历完。第二，题目限定不可重复，很容易想到一个对应的数据结构——set。具体思路是遍历字符串，检查当前字符串是否在set里面，如果不存在则插入set，并且将当前set的长度与最长串的长度作比较，取最大值；如果set里已存在该字符，基于set结构的不重复特性，逐渐从开头删除字符直到无相同字符。

解题代码：

class Solution {
public:
  int lengthOfLongestSubstring(string s)
  {
    set<char> t;
    int res = 0;
    for (int i = 0,j = 0; i < s.size(); ++i)
    {
        if (t.find(s[i]) == t.end())
        {
            t.insert(s[i]);
            res = max(res, (int)t.size());
        }
        else
        {
            t.erase(s[j++]);
        }
    }
    return res;
  }
};