这是一道面经题目。
就是给你一个invalid parenthese, 删掉一些，输出其中一种valid的情况。
难度低于

1. Remove Invalid Parentheses
   http://www.jianshu.com/p/107a8e166351

My code:

import java.util.ArrayList;
import java.util.HashSet;

public class Solution {
    public String balance(String s) {
        if (s == null || s.length() == 0)
            return s;
        int left = 0;
        int right = 0;
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            char curr = s.charAt(i);
            if (curr == '(') {
                left++;
                sb.append('(');
            }
            else if (curr == ')' && left > right) {
                right++;
                sb.append(')');
            }
        }
        s = sb.toString();
        sb = new StringBuilder();
        left = 0;
        right = 0;
        for (int i = s.length() - 1; i >= 0; i--) {
            char curr = s.charAt(i);
            if (curr == ')') {
                right++;
                sb.append(')');
            }
            if (curr == '(' && right > left) {
                left++;
                sb.append('(');
            }
        }
        return sb.reverse().toString();
    }
    
    
    
    public static void main(String[] args) {
        Solution test = new Solution();
        System.out.println(test.balance("()())()"));
    }
}

Anyway, Good luck, Richardo!