Minimum Window Substring

public class Solution {
    public String minWindow(String s, String t) {
        if(s==null || s.length()==0)    return "";
        if(t==null || t.length()==0)    return "";
        int[] mark = new int[256];
        int[] valid = new int[256];
        for(int i=0; i<t.length(); i++)
        {
            mark[t.charAt(i)]++;
            valid[t.charAt(i)]++;
        }
        int minStart = 0;
        int minSize = Integer.MAX_VALUE;
        int start = 0;
        int count = 0;
        
        for(int end=0; end<s.length(); end++)
        {
            if(mark[s.charAt(end)] > 0)
            {
                valid[s.charAt(end)]--;
                if(valid[s.charAt(end)] >= 0) count++;
                if(count == t.length())
                {
                    while(true)
                    {
                        if(mark[s.charAt(start)] > 0)
                        {
                            if(valid[s.charAt(start)] < 0) valid[s.charAt(start)]++;
                            else break;
                        }
                        start++;
                    }
                    if(minSize > end-start+1)
                    {
                        minSize = end-start+1;
                        minStart = start;
                    }
                }
            }
            
        }
        if(minSize == Integer.MAX_VALUE) return "";
        return s.substring(minStart, minStart+minSize);
    }
}