[Java 101] 76. Minimum Window Substring: Set, HashMap

class Solution {
    public String minWindow(String s, String t) {
        String ret = null;

        Set<Character> charSet = t.chars().mapToObj(c -> (char) c)
                        .collect(Collectors.toSet());
        List<Integer> indexList = new ArrayList<>();
        Map<Character, Integer> need = new HashMap<>();
        for (int i=0; i<s.length(); i++) {
            Character c = s.charAt(i);
            if (charSet.contains(c)) {
                indexList.add(i);
            }
        }
        for (int i=0; i<t.length(); i++) {
            Character c = t.charAt(i);
            need.put(c, need.getOrDefault(c, 0) + 1);
        }

        int j = 0;
        for (int i=0; i<indexList.size(); i++) {
            Character c1 = s.charAt(indexList.get(i));
            need.put(c1, need.get(c1)-1);
            if (need.get(c1) == 0) {
                charSet.remove(c1);
            }
            Character c2 = s.charAt(indexList.get(j));
            while (need.get(c2) < 0) {
                need.put(c2, need.get(c2)+1);
                j += 1;
                c2 = s.charAt(indexList.get(j));
            }
            if (charSet.isEmpty() && 
            (ret == null || (indexList.get(i) - indexList.get(j) + 1) < ret.length())) {
                ret = s.substring(indexList.get(j), indexList.get(i)+1);
            }
        }
        if (ret == null) return "";
        return ret;
    }
}