Qiita Teams that are logged in
You are not logged in to any team

Log in to Qiita Team
Community
OrganizationAdvent CalendarQiitadon (β)
Service
Qiita JobsQiita ZineQiita Blog
0
Help us understand the problem. What is going on with this article?
@vc7

31. Next Permutation

More than 1 year has passed since last update.

I did't know what next permutation really is but I have found some explanations on Wikipedia, and it helped.

Quick Explanation

The explanation below is grab from next permutation section on Wikipedia.

cf. https://en.wikipedia.org/wiki/Permutation#Generation_in_lexicographic_order

  1. Find the largest index k such that a[k] < a[k+1]. If no such index exist, the permutation is the last permutation.
  2. Find the largest index l greater than k such that a[k] < a[l].
  3. Swap the value of a[k] with that of a[l].
  4. Reverse the sequence from a[k+1] up to and including th final element a[n].

Other Resources

Code

class Solution {
    func nextPermutation(_ nums: inout [Int]) {
        // I have created methods for each step
        let k = findLargestK(&nums) // ①
        if k >= 0 {
            let l = findLargestL(&nums, k) // ②
            swap(&nums, k, l) // ③
        }
        reverse(&nums, k + 1) // ④
    }

    /// Find the largest index k such that a[k] < a[k+1].
    /// If no such index exist, the permutation is the last permutation.
    private func findLargestK(_ nums: inout [Int]) -> Int {
        // We start from the back because we need to find a largest index.
        // -2 for we need an extra space for k+1
        var k = nums.count - 2

        while k >= 0 && nums[k] >= nums[k+1] {
            k -= 1
        }

        return k
    }

    /// Find the largest index l greater than k such that a[k] < a[l].
    private func findLargestL(_ nums: inout [Int], _ k: Int) -> Int {
        // We start from the back because we need to find a largest index.
        var l = nums.count - 1

        while l >= 0 && nums[k] >= nums[l] {
            l -= 1
        }

        return l
    }

    private func swap(_ nums: inout [Int], _ a: Int, _ b: Int) {
        let temp = nums[a]
        nums[a] = nums[b]
        nums[b] = temp
    }

    private func reverse(_ nums: inout [Int], _ start: Int) {
        var lower = start
        var upper = nums.count - 1

        while lower < upper {
            swap(&nums, lower, upper)
            lower += 1
            upper -= 1
        }
    }
}
0
Help us understand the problem. What is going on with this article?
Why not register and get more from Qiita?
  1. We will deliver articles that match you
    By following users and tags, you can catch up information on technical fields that you are interested in as a whole
  2. you can read useful information later efficiently
    By "stocking" the articles you like, you can search right away
vc7
iOS Developer from 🇹🇼

Comments

No comments
Sign up for free and join this conversation.
Sign Up
If you already have a Qiita account Login
0
Help us understand the problem. What is going on with this article?