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.

   public string MinWindow(string s, string t) {
        if(t==null || t == "" || s == "") return t;
        if(t.Length > s.Length) return "";
        int max =s.Length+1;
        var res ="";
        var hashtable = new Dictionary<char,int>();
        int min =s.Length;
        for(int i =0;i< t.Length;i++)
        {
            if(hashtable.ContainsKey(t[i])) hashtable[t[i]]++;
            else hashtable.Add(t[i],1);
        }
        
        for(int i = 0;i<=s.Length - t.Length;i++)
        {
            var exist = new Dictionary<char,int>();
            int count =0;
            for(int j = i;j<s.Length;j++)
            {
                if(max < j-i+1) break;
                if(hashtable.ContainsKey(s[j]))
                {
                    if(exist.ContainsKey(s[j]))
                    {
                        if(exist[s[j]] < hashtable[s[j]])
                        {
                            count++;
                            exist[s[j]]++;
                            if(count >= t.Length)
                            {
                                if(max > j-i+1) {res =s.Substring(i,j-i+1);max = j-i+1;}
                                break;
                            }
                        }
                    }
                    else
                    {
                        count++;
                        exist.Add(s[j],1);
                        if(count >= t.Length) 
                        {
                            if(max > j-i+1) {res =s.Substring(i,j-i+1);max = j-i+1;}
                                break;
                        }
                    }
                }
            }
        }
        return res;
    }

public string MinWindow(string s, string t) {
        if(t==null || t == "" ||s==mull|| s == "") return t;
        if(t.Length > s.Length) return "";
        int max =s.Length+1;
        var res ="";
        var hashtable = new Dictionary<char,int>();
        for(int i =0;i< t.Length;i++)
        {
            if(hashtable.ContainsKey(t[i])) hashtable[t[i]]++;
            else hashtable.Add(t[i],1);
        }
        
        int left =0;
        int right =-1;
        int count = 0;
        while(right<s.Length)
        {
            if(count == t.Length)
            {
                if(max > right - left + 1)
                {
                    max= right - left + 1;
                    res = s.Substring(left,right - left + 1);
                }
                if(hashtable.ContainsKey(s[left]))
                {
                    if(hashtable[s[left]] ==0)  count--;
                    hashtable[s[left]]++;
                   
                }
               
                left++;
            }
            else if(right<s.Length-1)
            {
                right++;
                if(hashtable.ContainsKey(s[right]) )
                {
                    if(hashtable[s[right]]>0) count++;
                    hashtable[s[right]]--;
                }
            }
            else right++;
            
        }
        return res;

    }