LeetCode-in-Swift

22. Generate Parentheses

Medium

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

Example 1:

Input: n = 3

Output: [”((()))”,”(()())”,”(())()”,”()(())”,”()()()”]

Example 2:

Input: n = 1

Output: [”()”]

Constraints:

• 1 <= n <= 8

To solve the “Generate Parentheses” problem in Swift with a Solution class, we can use a backtracking approach. Here are the steps:

1. Define a Solution class.
2. Define a method named generateParenthesis that takes an integer n as input and returns a list of strings representing all combinations of well-formed parentheses.
3. Create an empty list to store the result.
4. Call the recursive helper function generateParenthesisHelper with the empty string "", counts of open and close parentheses set to 0, the value of n, and the result list.
5. In the generateParenthesisHelper function:
   • If the length of the current string is equal to 2 * n, add it to the result list.
   • If the count of open parentheses is less than n, append an open parenthesis to the current string and call the function recursively with increased open count.
   • If the count of close parentheses is less than the count of open parentheses, append a close parenthesis to the current string and call the function recursively with increased close count.
6. Return the result list.

Here’s the implementation:

class Solution {
    func generateParenthesis(_ n: Int) -> [String] {
        var sb = ""
        var ans = [String]()
        generate(&sb, &ans, n, n)
        return ans
    }

    private func generate(_ sb: inout String, _ str: inout [String], _ open: Int, _ close: Int) {
        if open == 0 && close == 0 {
            str.append(sb)
            return
        }
        if open > 0 {
            sb.append("(")
            generate(&sb, &str, open - 1, close)
            sb.removeLast()
        }
        if close > 0 && open < close {
            sb.append(")")
            generate(&sb, &str, open, close - 1)
            sb.removeLast()
        }
    }
}

This implementation provides a solution to the “Generate Parentheses” problem in Swift using a backtracking approach.

This site is open source. Improve this page.