[LeetCode] 22. Generate Parentheses（括号生成器）

• Difficulty: Medium

• Related Topics: String, Backtracking

• Link: https://leetcode.com/problems/generate-parentheses/

Description

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.
给 n 对括号，写一个函数生成所有有效的括号对。

Examples

Example 1

Input: n = 3
Output: ["((()))","(()())","(())()","()(())","()()()"]

Example 2

Input: n = 1
Output: ["()"]

Constraints

• 1 <= n <= 8

Solution

在回溯类的题目中，此题属于排列树。考虑到本题要求括号对是有效的，自然一开始往里添加的时候，就要先添加左括号，然后如果右括号数量不够，尝试在其中添加右括号，代码如下：

class Solution {
    private val result = arrayListOf<String>()

    fun generateParenthesis(n: Int): List<String> {
        backtrack("", 0, 0, n)
        return result
    }

    private fun backtrack(cur: String, left: Int, right: Int, total: Int) {
        if (cur.length == total * 2) {
            result.add(cur)
            return
        }
        if (left < total) {
            backtrack("$cur(", left + 1, right, total)
        }
        if (right < left) {
            backtrack("$cur)", left, right + 1, total)
        }
    }
}