class Solution {

public:

string minWindow(string s, string t) {

//create the hash table for string t.

unordered_map<char, int> T;

unordered_map<char, bool> found;

int minLen= s.size()+1;

int minLeft=0,minRight;

for(int i = 0;i<t.size();i++)

{

T[t[i]]++;

found[t[i]] = true; //indicate the occurance.

}

//cout << "debug " << endl;

int count = 0;

for (int left=0,right=0;right<s.size();right++)

{

//get the char at end of string.

if(found[s[right]])//see right charcter in the T.

{

T[s[right]]--; //reduce the number of right character

if(T[s[right]]>=0) count++;

//cout << "debug 2" <<endl;

while(count == t.size())

{

if(right-left+1<minLen)

{

minLeft = left;

minRight = right;

minLen = right-left+1;

}

if(found[s[left]]) //see the left character in the T

{

T[s[left]]++;

if(T[s[left]]>0) count--; //it must be large than 0.

}

left++;

}

}

}

return minLen>s.size()?"":s.substr(minLeft,minLen);

}

};