LeetCode之Substring with Concatenation of All Words

/*本题和Minimum Window Substring题目很相似：都利用双指针在第一个数组中标志一个区间，
让这个区间满足第二个数组表示的某种属性，目的是在第一个数组中寻找出满足这种属性的区间。
源代码参考自：http://www.cnblogs.com/panda_lin/archive/2013/10/30/substring_with_concatenation_of_all_words.html*/
class Solution {
public:
	vector<int> findSubstring(string S, vector<string> &L) {
		std::vector<int> result;
 
		if(L.empty() || S.empty()) return result;
 
		int word_len(L[0].size()), letter_num(L.size()*L[0].size());
		if(S.size() < letter_num) return result;
 
		std::map<string, int> expected_count;
		for(int i = 0; i < L.size(); ++i){
			++expected_count[L[i]];
		}
		std::map<string, int> appeared_count;
		for(int i = 0; i <= S.size() - letter_num; ++i){
			appeared_count.clear();
			int j;
			for(j = 0; j < L.size(); ++j){
				std::string word = S.substr(i + j*word_len, word_len);
				if(expected_count.find(word) != expected_count.end()){
					++appeared_count[word];
					if(appeared_count[word] > expected_count[word]) break;
				}
				else{
					break;
				}
			}
			if(j == L.size()) result.push_back(i);
		}
 
		return result;
	}
};