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GenerateParentheses.java

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/*

Author: King, wangjingui@outlook.com

Date: Dec 20, 2014

Problem: Generate Parentheses

Difficulty: Medium

Source: https://oj.leetcode.com/problems/generate-parentheses/

Notes:

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:

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

Solution: Place n left '(' and n right ')'.

Cannot place ')' if there are no enough matching '('.

*/

public class Solution {

public List<String> generateParenthesis(int n) {

ArrayList<String> res = new ArrayList<String>();

generateParenthesisRe(res, n, n, "");

return res;

}

public void generateParenthesisRe(ArrayList<String> res, int left, int right, String s) {

if (left == 0 && right == 0)

res.add(s);

if (left > 0)

generateParenthesisRe(res, left - 1, right, s + "(");

if (right > left)

generateParenthesisRe(res, left, right - 1, s + ")");

}

}