[LeetCode] Longest Substring Without Repeating Characters

在後面發現重複的字符後，就把前面的重複字符及之前的全部去掉，像毛毛蟲爬行一樣，同時統計最大的子序列長度。

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
    	if (s.size() < 2) {
    		return s.size();
    	}
    	
    	int length = 0;
    	for(int first = 0, last = 1; last < s.size(); ++last) {
    		string sub = s.substr(first, last - first);
    		string::size_type dup = sub.find(s[last]);
    		if (dup != string::npos ){
    			first += (dup + 1);
    		}
    		
    		int gap = last - first + 1;
    		length = (length >= gap ? length : gap);
    	}
    	return length;  
    }
};