【LeetCode】Generate Parentheses

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

For example, given n = 3, a solution set is:

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

java code : 采用递归树的思想，当左括号数大于右括号数时可以加左或者右括号，否则只能加左括号，当左括号数达到n时，剩下全部

加右括号。

public class Solution {
    public ArrayList<String> generateParenthesis(int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        ArrayList<String> res = new ArrayList<String>();
        generate(res, "", 0, 0, n);
        return res;
    }
    public void generate(ArrayList<String> res, String tmp, int lhs, int rhs, int n)
    {
        if(lhs == n)
        {
            for(int i = 0; i < n - rhs; i++)
			{
				tmp += ")";
			}
			res.add(tmp);
			return ;
        }
        generate(res, tmp + "(", lhs + 1, rhs, n);
        if(lhs > rhs)
            generate(res, tmp + ")", lhs, rhs + 1, n);
    }
}