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题目：
You are given a string, s, and a list of words, words, that are all of the same length. Find all starting indices of substring(s) in s that is a concatenation of each word in words exactly once and without any intervening characters.
For example, given:
s: "barfoothefoobarman"
words: ["foo", "bar"]
You should return the indices: [0,9].
(order does not matter).
代码：
class Solution { public: vector<int> findSubstring(string s, vector<string>& words) { vector<int> ret; if ( words.empty() || s.empty() ) return ret; const int len_w = words[0].size(); // length of substring in words const int end = words.size()*len_w; // total length of words if ( end>s.size() ) return ret; // s size not enough map<string, int> ori; for ( int i=0; i<words.size(); ++i ) { if ( ori.find(words[i])==ori.end() ){ ori[words[i]] = 1; } else{ ori[words[i]]++; } } map<string, int> found; int match_num = 0; int begin = 0; int i = 0; while ( i<s.size() ) { //cout << "i:" << i << endl; //cout << "m:" << match_num << endl; //cout << "begin:" << begin << endl; // match one substring in words if ( ori.find(s.substr(i,len_w))!=ori.end() ) { found[s.substr(i,len_w)]++; // substring occur more than times in words if ( found[s.substr(i,len_w)]>ori[s.substr(i,len_w)] ) { found.clear(); match_num = 0; begin++; i=begin; continue; } i = i+len_w; match_num++; //cout << match_num << endl; // match all substrings in words and push back a starting indices if ( match_num==words.size() ) { ret.push_back(i-end); found.clear(); begin++; i = begin; match_num=0; } } // not match else { found.clear(); match_num = 0; begin++; i=begin; } } return ret; } };
tips：
采用双指针技巧。
维护几个关键变量：
1. begin：可能的有效开始位置
2. match_num: 已经匹配上的words中的个数
3. ori: words中每个string，及其出现的次数
4. found: 当前已匹配的interval中，words中每个string，及其出现的次数
思路就是如果找到了匹配的某个string,i就不断向后跳；跳的过程中可能有两种情况不能跳了：
a) 下一个固定长度的子字符串不匹配了
b) 下一个固定长度的子字符串虽然匹配上了words中的一个，但是匹配的数量已经超过了ori中该string的数量
如果一旦不能跳了，就要把match_num和found归零；而且需要回溯到begin++的位置，开始新一轮的搜寻。
这里有个细节要注意：
直接用found.clear()就可以了，如果一个一个清零，会超时。
【Substring with Concatenation of All Words】cpp
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原文地址：http://www.cnblogs.com/xbf9xbf/p/4562202.html
