Smallest Window in a string containing all the characters of another string

//{ Driver Code Starts
#include<bits/stdc++.h>
using namespace std;


// } Driver Code Ends
class Solution
{
    public:

    string smallestWindow (string s, string p)
    {
        string str=s;
        string pat=p;
        int no_of_chars=256;
          int len1 = str.length();
    int len2 = pat.length();

    if (len1 < len2) {

        return "-1";
    }

    int hash_pat[no_of_chars] = { 0 };
    int hash_str[no_of_chars] = { 0 };

    for (int i = 0; i < len2; i++)
        hash_pat[pat[i]]++;

    int start = 0, start_index = -1, min_len = INT_MAX;


    int count = 0;
    for (int j = 0; j < len1; j++) {


        hash_str[str[j]]++;

        if (hash_str[str[j]] <= hash_pat[str[j]])
            count++;


        if (count == len2) {

            while (hash_str[str[start]]
                       > hash_pat[str[start]]
                   || hash_pat[str[start]] == 0) {

                if (hash_str[str[start]]
                    > hash_pat[str[start]])
                    hash_str[str[start]]--;
                start++;
            }

            int len_window = j - start + 1;
            // cout<<len_window<<endl;
            if (min_len > len_window) {
                min_len = len_window;
                start_index = start;
            }
        }
    }

    if (start_index == -1) {

        return "-1";
    }

    return str.substr(start_index, min_len);
    }
};

//{ Driver Code Starts.
int main()
 {
    int t;
    cin>>t;
    while(t--)
    {
        string s;
        cin>>s;
        string pat;
        cin>>pat;
        Solution obj;
        cout<<obj.smallestWindow(s, pat)<<endl;

    }
    return 0;
}
// } Driver Code Ends