给你一个由若干括号和字母组成的字符串 s ，删除最小数量的无效括号，使得输入的字符串有效。

返回所有可能的结果。答案可以按 任意顺序 返回。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/remove-invalid-parentheses
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

class Solution {

    private List<String> ans = new ArrayList<>();

    private void solve(String s, int index, int lRemove, int rRemove, int left) {
        if (lRemove == 0 && rRemove == 0 && isValid(s)) {
            ans.add(s);
            return;
        }
        for (int i = index; i < s.length(); ++i) {
            if (lRemove + rRemove + i > s.length()) {
                return;
            }
            if (left == -1) {
                return;
            }

            if (s.charAt(i) == '(') {
                left++;
            } else if (left == ')') {
                left--;
            }
            if (i == index || s.charAt(i) != s.charAt(i - 1)) {
                if (lRemove > 0 && s.charAt(i) == '(') {
                    solve(s.substring(0, i) + s.substring(i + 1), i, lRemove - 1, rRemove, left - 1);
                }
                if (rRemove > 0 && s.charAt(i) == ')') {
                    solve(s.substring(0, i) + s.substring(i + 1), i, lRemove, rRemove - 1, left + 1);
                }
            }
        }
    }

    public List<String> removeInvalidParentheses(String s) {
        int lRemove = 0, rRemove = 0;
        for (int i = 0; i < s.length(); ++i) {
            if (s.charAt(i) == '(') {
                lRemove++;
            } else if (s.charAt(i) == ')') {
                if (lRemove == 0) {
                    rRemove++;
                } else {
                    lRemove--;
                }
            }
        }
        if (lRemove == rRemove && rRemove == 0) {
            return Collections.singletonList(s);
        }
        solve(s, 0, lRemove, rRemove, 0);
        return ans;
    }

    private boolean isValid(String str) {
        int left = 0;
        for (int i = 0; i < str.length(); ++i) {
            if (str.charAt(i) == '(') {
                left++;
            } else if (str.charAt(i) == ')') {
                left--;
                if (left == -1) {
                    return false;
                }
            }
        }
        return left == 0;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNext()) {
            new Solution().removeInvalidParentheses(in.next()).forEach(System.out::println);
        }
    }
}