leetcode301

 class Solution {
     public List<String> removeInvalidParentheses(String s) {
         List<String> ans = new ArrayList<>();
         remove(s, ans, 0, 0, new char[]{'(', ')'});
         return ans;
     }

     public void remove(String s, List<String> ans, int last_i, int last_j,  char[] par) {
         for (int stack = 0, i = last_i; i < s.length(); ++i) {
             if (s.charAt(i) == par[0]) stack++;
             if (s.charAt(i) == par[1]) stack--;
             if (stack >= 0) continue;
             for (int j = last_j; j <= i; ++j)
                 if (s.charAt(j) == par[1] && (j == last_j || s.charAt(j - 1) != par[1]))
                     remove(s.substring(0, j) + s.substring(j + 1, s.length()), ans, i, j, par);
             return;
         }
         String reversed = new StringBuilder(s).reverse().toString();
         if (par[0] == '(') // finished left to right
             remove(reversed, ans, 0, 0, new char[]{')', '('});
         else // finished right to left
             ans.add(reversed);
     }
 }

参考：https://leetcode.com/problems/remove-invalid-parentheses/discuss/75027/Easy-Short-Concise-and-Fast-Java-DFS-3-ms-solution