Minimum Window Substring

Minimum Window Substring
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”.

题目的要求是寻找最小覆盖子串，如果找不到返回”“。在解题过程中，我们可以利用hash的思想，用thash[T[i]]来存储，字符串T中各个单词出现的次数。

class Solution {public:    string minWindow(string s, string t) {        vector<int> shash(128,0);        vector<int> thash(128,0);        for(int i=0;i<t.size();i++){            thash[t[i]]++;        }        int beg=-1,end=s.size(),start=0,found=0;        int min_len=s.size();        for(int i=0;i<s.size();i++){            shash[s[i]]++;            if(shash[s[i]]<=thash[s[i]]){//通过found来判断是否找到了所有t中的单词                found++;            }            if(found==t.size()){                while(start<i&&shash[s[start]]>thash[s[start]]){   //过滤掉字符串s中不属于t的字符                    shash[s[start]]--;    //或者属于t中的字符，但不是坐标最大的那个，如bba,ba取第二个b                    start++;                }                if(i-start+1<=min_len){                   beg=start;                   end=i;                   min_len=i-start+1;                }                shash[s[start]]--;                start++;                found--;            }        }        if(beg==-1)            return "";        else            return s.substr(beg,end-beg+1);    }};