Problem

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

Example

Given n = 3, a solution set is:

"((()))", "(()())", "(())()", "()(())", "()()()"

Note

By the way,

Solution

public class Solution {    // @param n n pairs    // @return All combinations of well-formed parentheses    public ArrayList
    
      generateParenthesis(int n) {        // Write your code here        ArrayList
     
       res = new ArrayList
      
       ();        helper(n, n, new String(), res);        return res;    }    public void helper(int l, int r, String s, List
       
         res) {                if (r < l) return; //限定条件：永远先放左括号，剩余的右括号要比左括号多        if (l > 0) {            helper(l-1, r, s+"(", res);        }        if (r > 0) {            helper(l, r-1, s+")", res);        }        if (l == 0 && r == 0) { //放完了所有的括号，把s加入res            res.add(s);        }    }}