最小覆盖子串

/*
 * 思路：
 * 使用滑动窗口方法，并结合Java Stream API来进行一些操作。
 * 使用Stream API来初始化目标字符频率表，
 * 其他逻辑与普通Java实现类似，但不强求使用Stream API处理整个问题。
 */

import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class Solution {
    public String minWindow(String s, String t) {
        if (s == null || t == null || s.length() < t.length()) {
            return "";
        }

        Map<Character, Integer> targetFreq = t.chars().boxed()
            .collect(Collectors.toMap(k -> (char) k.intValue(), v -> 1, Integer::sum));

        int left = 0, right = 0;
        int required = targetFreq.size();
        int formed = 0;
        Map<Character, Integer> windowCounts = new HashMap<>();
        int[] ans = {-1, 0, 0};

        while (right < s.length()) {
            char c = s.charAt(right);
            windowCounts.put(c, windowCounts.getOrDefault(c, 0) + 1);

            if (targetFreq.containsKey(c) && windowCounts.get(c).intValue() == targetFreq.get(c).intValue()) {
                formed++;
            }

            while (left <= right && formed == required) {
                c = s.charAt(left);
                if (ans[0] == -1 || right - left + 1 < ans[0]) {
                    ans[0] = right - left + 1;
                    ans[1] = left;
                    ans[2] = right;
                }

                windowCounts.put(c, windowCounts.get(c) - 1);
                if (targetFreq.containsKey(c) && windowCounts.get(c).intValue() < targetFreq.get(c).intValue()) {
                    formed--;
                }
                left++;
            }
            right++;
        }

        return ans[0] == -1 ? "" : s.substring(ans[1], ans[2] + 1);
    }
}