Minimum Window Substring, 包含子串的最小窗口，双指针

public class MinWindowSubstring
{
	public String minWindow(String s, String t) {
	    if(t.length()>s.length()) 
	        return "";
	    String result = "";
	 
	    //统计t中每个字符的个数
	    HashMap<Character, Integer> target = new HashMap<Character, Integer>();
	    for(int i=0; i<t.length(); i++)
	    {
	        char c = t.charAt(i);    
	        if(target.containsKey(c))
	        {
	            target.put(c,target.get(c)+1);
	        }
	        else
	        {
	            target.put(c,1);  
	        }
	    }
	 
	    //map统计t中字符在s中的个数
	    HashMap<Character, Integer> map = new HashMap<Character, Integer>();
	    int left = 0;//窗口左指针
	    int minLen = s.length()+1;//最小窗口
	 
	    int count = 0; // 统计s中包含t中元素数
	 
	    for(int i=0; i<s.length(); i++)
	    {
	        char c = s.charAt(i);
	 
	        if(target.containsKey(c))
	        {
	            if(map.containsKey(c))
	            {
	                if(map.get(c)<target.get(c))
	                {
	                    count++;
	                }
	                map.put(c,map.get(c)+1);
	            }
	            else
	            {
	                map.put(c,1);
	                count++;
	            }
	        }//target.containsKey(c)
	 
	        if(count == t.length())//当前窗口包含所有t中元素
	        {
	            char sc = s.charAt(left);
	            //从左边开始滑动窗口
	            while (!map.containsKey(sc) || map.get(sc) > target.get(sc)) 
	            {
	                if (map.containsKey(sc) && map.get(sc) > target.get(sc))
	                {
	                	map.put(sc, map.get(sc) - 1);
	                }
	                
	                left++;
	                
	                sc = s.charAt(left);
	            }
	            //计算最小窗口
	            if (i - left + 1 < minLen) 
	            {
	                result = s.substring(left, i + 1);
	                minLen = i - left + 1;
	            }
	        }//count == t.length
	        
	    }//for
	 
	    return result;
	}
}