Leetcode 76 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".

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.

Solution 1:  Use a counter to store the count number of each character. C1 is for S and C2 is for T, use C1-C2 and maintain a slide window that count is non-negative.

 public class Solution {  
   public String minWindow(String s, String t) {  
     int m=s.length();  
     int n=t.length();  
     if (m==0 || n==0) return "";  
     int[] c1=new int[256];  
     int[] c2=new int[256];  
     for (int i=0; i<m; i++) c1[s.charAt(i)]++;  
     for (int i=0; i<n; i++) c2[t.charAt(i)]++;  
     for (int i=0; i<256; i++) {  
       c1[i]-=c2[i];  
       if (c1[i]<0) return "";  
     }  
     int i=0, j=m-1, min=m;  
     String res=s;  
     while (c1[s.charAt(j)]>0) c1[s.charAt(j--)]--;  
     while (true) {  
       while (c1[s.charAt(i)]>0) c1[s.charAt(i++)]--;  
       if (j-i+1<min) {  
         min=j-i+1;  
         res=s.substring(i,j+1);  
       }  
       if (++j==m) break;  
       c1[s.charAt(j)]++;  
     }  
     return res;  
   }  
 }