题目
思路参考《算法小抄》

class Solution {
    public String minWindow(String s, String t) {
        int startIndex = -1, endIndex = s.length(), valid = 0, left = 0, right = 0;
        char[] sArray = s.toCharArray();
        char[] tArray = t.toCharArray();
        int[] need = new int[256];
        int[] window = new int[256];
        Set<Character> set = new HashSet<>();
        for (char c : tArray) {
            set.add(c);
            ++need[c];
        }
        while (right < sArray.length) {
            char c = sArray[right];
            ++right;
            if (!set.contains(c)) {
                continue;
            }
            ++window[c];
            if (window[c] == need[c]) {
                ++valid;
            }
            while (valid == set.size() && left < right) {
                if (right - left < endIndex - startIndex) {
                    startIndex = left;
                    endIndex = right;
                }
                char d = sArray[left];
                ++left;
                if (!set.contains(d)) {
                    continue;
                } else {
                    if (window[d] == need[d]) {
                        --valid;
                    }
                    --window[d];
                }
            }
        }

        return startIndex == -1 ? "" : s.substring(startIndex, endIndex);
    }
}