Smallest window containing all characters

import java.util.*;

class Solution {
    public static String smallestWindow(String s, String p) {
        int n = s.length();
        int m = p.length();
        Map<Character, Integer> mp = new HashMap<>();
        for (int i = 0; i < m; i++) {
            char c = p.charAt(i);
            mp.put(c, mp.getOrDefault(c, 0) + 1);
        }
        int cnt = mp.size();
        int i = 0;
        int j = 0;
        int len = Integer.MAX_VALUE;
        int st = -1;
        while (j < n) {
            char c = s.charAt(j);
            if (mp.containsKey(c)) {
                mp.put(c, mp.get(c) - 1);
                if (mp.get(c) == 0)
                    cnt--;
                while (cnt == 0) {
                    if (len > j - i + 1) {
                        len = j - i + 1;
                        st = i;
                    }

                    char ch = s.charAt(i);
                    if (mp.containsKey(ch)) {
                        mp.put(ch, mp.get(ch) + 1);
                        if (mp.get(ch) == 1)
                            cnt++;
                    }
                    i++;
                }
            }
            j++;
        }

        if (st == -1)
            return "";

        return s.substring(st, st + len);
    }
}