301. Remove Invalid Parentheses

Remove the minimum number of invalid parentheses in order to make the input string valid. Return all possible results.
Note: The input string may contain letters other than the parentheses ( and ).
Examples:
"()())()" -> ["()()()", "(())()"]
"(a)())()" -> ["(a)()()", "(a())()"]
")(" -> [""]

Solution：dfs recursive

思路：
https://discuss.leetcode.com/topic/34875/easy-short-concise-and-fast-java-dfs-3-ms-solution
Time Complexity: O() Space Complexity: O()

Solution Code：

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 wrong ')'
        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;
        }
        
        // for wrong '('
        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);
        }
    }
}