Minimum Window Substring.txt

problem：
Given a string S and a string T, find the minimum window in S which will contain all the characters in T in complexity O(n).

For example,
S = "ADOBECODEBANC"
T = "ABC"

Minimum window is "BANC".

Note:
If there is no such window in S that covers all characters in T, return the emtpy string "".

If there are multiple such windows, you are guaranteed that there will always be only one unique minimum window in S.

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solution:
//字符哈希+双指针
//尾指针不断往后扫，当扫到有一个窗口包含了所有T的字符，然后再收缩头指针，直到不能再收缩为止。
//最后记录所有可能的情况中窗口最小的
string minWindow(string S, string T)
{
  int nS = S.size();
	int nT = T.size();
	if(nS == 0 || nT == 0)
		return "";
	int needToFind[256] = {0};
	int hasFound[256] = {0};
	int minBegin, minEnd;
	int minWindow = nS + 1;
	int count = 0;
	for(int i = 0; i < nT; ++i)
		needToFind[T[i]]++;
	int begin = 0, end = 0;
	for(; end < nS; ++end)
	{
		char ch = S[end];
		hasFound[ch]++;
		if(hasFound[ch] <= needToFind[ch])
			count++;
		if(count == nT)
		{
			while(needToFind[S[begin]] == 0 || hasFound[S[begin]] > needToFind[S[begin]])
			{
				if(hasFound[S[begin]] > needToFind[S[begin]])
					hasFound[S[begin]]--;
				begin++;
			}
			int length = end - begin + 1;
			if(length < minWindow)
			{
				minBegin = begin;
				minEnd = end;
				minWindow = length;
			}
		}
	}
	return minWindow <= nS ? S.substr(minBegin, minWindow) : "";
}