Minimum Window Substring

Given a string and a pattern, find the smallest substring in the given string which has all the character occurrences of the given pattern.

Example 1:

• Input: s = "ADOBECODEBANC", t = "ABC"
• Output: "BANC"

 class Solution {
    public String minWindow(String s, String t) {
        int windowStart = 0, matched = 0;
        var out = "";
        var charMap = new HashMap<Character, Integer>();
        for (var i = 0; i < t.length(); i ++) {
            var ch = t.charAt(i);
            charMap.put(ch, charMap.getOrDefault(ch, 0) + 1);
        }
        for (var windowEnd = 0; windowEnd < s.length(); windowEnd ++) {
            var chEnd = s.charAt(windowEnd);
            if (charMap.containsKey(chEnd)) {
                charMap.put(chEnd, charMap.get(chEnd) - 1);
                if (charMap.get(chEnd) == 0) {
                    matched ++;
                }
            }
            while (matched == charMap.size() && windowStart < s.length()) {
                if (windowEnd - windowStart + 1 < out.length() || out.isEmpty()) {
                    out = s.substring(windowStart, windowEnd + 1);
                }
                var chStart = s.charAt(windowStart);
                if (charMap.containsKey(chStart)) {
                    if (charMap.get(chStart) == 0) {
                        matched --;
                    }
                    charMap.put(chStart, charMap.get(chStart) + 1);
                }
                windowStart ++;
            }
        }
        return out;
    }
}