LC 76.java

```java


  public String minimumWindow01(String s, String t) {
        int l = 0, r = 0, N = s.length() , start = 0, minLen = Integer.MAX_VALUE;
        Map<Character, Integer> needs = new HashMap();
        for (char c : t.toCharArray()) {
            needs.put(c, needs.getOrDefault(c, 0) + 1);
        }
        Map<Character, Integer> windows = new HashMap();
        int match = 0;
        while (r < N) {
            char c1 = s.charAt(r);
            if (needs.containsKey(c1)) {
                windows.put(c1, windows.getOrDefault(c1, 0) + 1);
                if (windows.get(c1) == needs.get(c1)) {
                    match++;
                }
            }
            r++;

            // find 可行解, pursue 最优解
            while (match == needs.size()) {
                // update
                if (r - l < minLen) {
                    start = l;
                    minLen = r - l;
                }
                char c2 = s.charAt(l);
                if (needs.containsKey(c2)) {
                    windows.put(c2, windows.get(c2) - 1);
                    if (windows.get(c2) < needs.get(c2)) {
                        match--;
                    }
                }
                l++;
            }
        }
        return minLen == Integer.MAX_VALUE ? "" : s.substring(start, start + minLen);
    }


```