【Java】力扣 76. 最小覆盖子串

一、问题

最小覆盖子串

二、解题代码

        public String minWindow(String s, String t) {
            if (t.length() > s.length()) {
                return "";
            }
            HashMap<Character, Integer> window = new HashMap<>();
            HashMap<Character, Integer> need = new HashMap<>();
            for (int i = 0; i < t.length(); i++) {
                need.put(t.charAt(i), need.getOrDefault(t.charAt(i), 0) + 1);
            }
            int left = 0, right = 0;
            //valid 变量表示窗口中满足 need 条件的字符个数
            int valid = 0;
            //最小子串开始位置
            int start = 0;
            //最小子串长度
            int len = Integer.MAX_VALUE;
            while (right < s.length()) {
                char add = s.charAt(right);
                //增大窗口
                right++;
                if (need.containsKey(add)) {
                    //字符存在更新窗口次数
                    window.put(add, window.getOrDefault(add, 0) + 1);
                    if (window.get(add).equals(need.get(add))) {
                        //当字符出现在窗口时增加满足 need 条件的字符个数
                        valid++;
                    }
                }
                // 判断左侧窗口是否要收缩
                while (valid == need.size()) {
                    if (right - left < len) {
                        // 在这里更新最小覆盖子串
                        len = right - left;
                        start = left;
                    }
                    //要移出窗口的字符
                    char remove = s.charAt(left);
                    //缩小窗口
                    left++;
                    if (need.containsKey(remove)) {
                        if (window.get(remove).equals(need.get(remove))) {
                            valid--;
                        }
                        window.put(remove, window.get(remove) - 1);
                    }
                }

            }
            return len == Integer.MAX_VALUE ? "" : s.substring(start, start + len);
        }