leetcode Minimum Window Substring答案解析

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 empty string

""

.

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

要求查找S中包含T中所有字母的最短子串

解题思路：

建立两个索引表，分别用来存储T中每个字母出现的次数（即needfound）和已经查找过的在start到end中间的各字母的出现次数

class Solution {

public:

    string minWindow(string s, string t) {

        vector<int> found(128,0);

        vector<int> tt(128,0);

        for(int i=0;i<t.length();i++)

            tt[t[i]]++;

        found[s[0]]++;

        int count=t.length(),start=0,end=0,ans=s.length()+1,minbegin=0;

        if(tt[s[0]]>=found[s[0]])

            count--;

        while(end<s.length()){

            if(count==0){

                while(found[s[start]]>tt[s[start]]){

                    found[s[start]]--;

                    start++;

                }

                if(end-start+1<ans){

                    ans=end-start+1;

                    minbegin=start;

                }

            }

            end++;

            found[s[end]]++;

            if(tt[s[end]]>=found[s[end]])

                count--;

        }

        if(ans>s.length())

            return "";

        return s.substr(minbegin,ans);

    }

};