Minimum Window Substring

public class Solution {    public String minWindow(String s, String t) {        if (s == null || s.length() == 0) {            return "";        }        Map<Character, Integer> map = new HashMap<>();        for (int i = 0; i < t.length(); i++) {            char c = t.charAt(i);            if (map.containsKey(c)) {                map.put(c, map.get(c) + 1);            } else {                map.put(c, 1);            }        }        int prev = 0, count = 0, minLen = s.length() + 1;        String res = "";        for (int i = 0; i < s.length(); i++) {            char c = s.charAt(i);            if (map.containsKey(c)) {                map.put(c, map.get(c) - 1);                if (map.get(c) >= 0) {                    count++;                }                while (count == t.length()) {                    char p = s.charAt(prev);                    if (map.containsKey(p)) {                        map.put(p, map.get(p) + 1);                        if (map.get(p) > 0) {                            count--;                            int len = i - prev + 1;                            if (len < minLen) {                                minLen = len;                                res = s.substring(prev, i + 1);                            }                        }                    }                    prev++;                }            }        }        return res;    }}