1 class Solution {
2 public List<String> removeInvalidParentheses(String s) {
3 List<String> ans = new ArrayList<>();
4 remove(s, ans, 0, 0, new char[]{'(', ')'});
5 return ans;
6 }
7
8 public void remove(String s, List<String> ans, int last_i, int last_j, char[] par) {
9 for (int stack = 0, i = last_i; i < s.length(); ++i) {
10 if (s.charAt(i) == par[0]) stack++;
11 if (s.charAt(i) == par[1]) stack--;
12 if (stack >= 0) continue;
13 for (int j = last_j; j <= i; ++j)
14 if (s.charAt(j) == par[1] && (j == last_j || s.charAt(j - 1) != par[1]))
15 remove(s.substring(0, j) + s.substring(j + 1, s.length()), ans, i, j, par);
16 return;
17 }
18 String reversed = new StringBuilder(s).reverse().toString();
19 if (par[0] == '(') // finished left to right
20 remove(reversed, ans, 0, 0, new char[]{')', '('});
21 else // finished right to left
22 ans.add(reversed);
23 }
24 }
参考：https://leetcode.com/problems/remove-invalid-parentheses/discuss/75027/Easy-Short-Concise-and-Fast-Java-DFS-3-ms-solution