[Leetcode] Minimum Window Substring

 1 public class Solution {
 2     public String minWindow(String S, String T) {
 3         int N = S.length();
 4         int M = T.length();
 5         if(N < M)
 6             return "";
 7         int[] need=new int[256];
 8         int[] find=new int[256];
 9         for(int i=0;i<M;++i)
10             need[T.charAt(i)]++;
11         
12         int count=0;
13         int resStart=-1;
14         int resEnd=N;
15         
16         for(int start=0,end=0;end<N;++end){
17             if(need[S.charAt(end)]==0)
18                 continue;
19             if(find[S.charAt(end)]<need[S.charAt(end)])
20                 count++;
21             find[S.charAt(end)]++;
22             if(count!=M)
23                 continue;
24             
25             for(;start<end;++start){
26                 if(need[S.charAt(start)]==0)
27                     continue;
28                 if(find[S.charAt(start)]<=need[S.charAt(start)])
29                     break;
30                 find[S.charAt(start)]--;
31             }
32             if(end-start<resEnd-resStart){
33                 resStart=start;
34                 resEnd=end;
35             }
36         }
37         return (resStart==-1)?"":S.substring(resStart, resEnd+1);
38     }
39 }