[LeetCode 22] Generate Parentheses

Question

link

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:

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

Stats

Frequency	4
Diffficulty	3
Adjusted Difficulty	2
Time to use	----------

Ratings/Color = 1(white) 2(lime) 3(yellow) 4/5(red)

Solution

Very standard permutation search. You can read more from this link.

My code

public class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> ans = new ArrayList<String>();
        if (n == 0) {
            return ans;
        }
        helper(ans, "", n, n);
        return ans;
    }

    private void helper(List<String> ans, String path, int left, int right) {
        if (left == 0 && right == 0) {
            ans.add(path);
            return;
        }
        // add either left or right parenthese
        if (left > 0) {
            helper(ans, path + "(", left - 1, right);
        }
        if (right > left) {
            helper(ans, path + ")", left, right - 1);
        }
    }
}