C++ code for follow-up question (Touch every character only Once!!!)

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    The general idea is to group k strings into 26 buckets, each of which includes candidate strings with the same "next" character that needs to be compared with "next" character of t. Here comes code first:

    vector<bool> isSubsequence(vector<string> ss, string t) {
    	vector<string::const_iterator> iters(ss.size());
    	vector<vector<int> > waitingList(26);
    	int unfinished = ss.size();
    	for(int i = 0; i < ss.size(); i++) {
    		iters[i] = ss[i].begin();
    		if(iters[i] != ss[i].end()) waitingList[*iters[i] - 'a'].push_back(i);
    	for(char c : t) {
    		vector<int> updateList = waitingList[c - 'a'];
    		waitingList[c - 'a'].clear();
    		for(int i : updateList) {
    			if(iters[i] != ss[i].end()) waitingList[*iters[i] - 'a'].push_back(i);
    			else unfinished--;
    		if(unfinished == 0) break;
    	vector<bool> ans(ss.size());
    	for(int i = 0; i < ss.size(); i++) {
    		ans[i] = iters[i] == ss[i].end();
    	return ans;

    The idea is straightforward. The original idea of this solution should attribute to @haiwei624 . I just made several improvement and test a little.

    1. We should check if an iterator has reached the end of s;
    2. We should consider corner case when s is empty;
    3. Also, we can maintain a counter to record the number of s whose comparison is unfinished. If all s have finished their comparison, we can jump out of the loop. This can be helpful when strings in ss are similar, and t is extremely long, however, the worst case time complexity is still O(len(t) + sum(len(s[i]))). BTW, I think I agree with your time complexity, because every character can and can only be touched once, so that's why, right?

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