LeetCode-3. Longest Substring Without Repeating Characters(没有重复字母的最长子序列）

LeetCode-3. Longest Substring Without Repeating Characters

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.

思路：本题要求求解最大子序列（无重复字母）的长度，可以利用unordered_map存储字母来检查是否出现重复。用start来标记有效序列的起点，用ret来表示序列的最大子序列的长度。注意更新序列有效起点的表示为

	start = found->second+1 ;

另外还要注意出现“abba”这种情况，该情况会改变有效起点，因为当查找到第二个a时，起点不能变为第一个b,而应该直接将a加入，继续向后读，因此加上条件

found->second >= start

class Solution {
public:
	int lengthOfLongestSubstring(string s)
	{
		unordered_map<char, size_t> trace;
		size_t ret = 0, start = 0;
		for (size_t i = 0; i < s.size(); i++)
		{
			auto found = trace.find(s[i]);
			if (found != trace.end() && found->second >= start)
			{
				ret = max(ret, i -start);
				start = found->second+1 ;
			}
			trace[s[i]] = i;
		}

		return max(ret, s.size() - start);
	}
};