天题系列 Minimum Window Substring

public class Solution {
    public String minWindow(String S, String T) {
        // 讲解http://articles.leetcode.com/2010/11/finding-minimum-window-in-s-which.html
        //代码 http://www.cnblogs.com/huntfor/p/3915288.html
        String res = "";
        if(S==null || T==null) return "";
        
        int[]  found= new int[256];
        int[] needfind = new int[256];
        int minLen = S.length()+1;
        
        for(int i=0;i<T.length();i++){
            needfind[T.charAt(i)]++;
        }
        int left = 0, cnt =0;
        
        for(int i=0;i<S.length(); i++){ // i is right end
            if(needfind[S.charAt(i)]==0) continue;  // skip
            
            found[S.charAt(i)]++;
            if(found[S.charAt(i)] <= needfind[S.charAt(i)])
                cnt++;
                
            // while restraint reachs
            if(cnt==T.length()){
                while(left<S.length() &&(needfind[S.charAt(left)] ==0 || found[S.charAt(left)] >needfind[S.charAt(left)]) ){
                    if(found[S.charAt(left)] >needfind[S.charAt(left)])
                        found[S.charAt(left)]--;
                    
                    left++;
                }
                
                int winlen = i-left+1;
                if(winlen<minLen){
                    minLen = winlen;
                    res = S.substring(left, i+1);
                }
            }
        }
        
        return res;
    }
}