/*     * 301. Remove Invalid Parentheses     * 2016-7-3 by Mingyang     *https://discuss.leetcode.com/topic/34875/easy-short-concise-and-fast-java-dfs-3-ms-solution     */    public List
     
       removeInvalidParentheses(String s) {        List
      
        ans = new ArrayList<>();        remove(s, ans, 0, 0, new char[]{'(', ')'});        return ans;    }    public void remove(String s, List
       
         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);    }