Find the next permutation of a number in Go (Golang)

Overview

The objective is to find the next permutation of a given number based upon lexicographical sorting order. If the next permutation is not possible then return the same number

For eg

Input: [1,2,3]
Output: [1,3,2]

Input: [2,1]
Output: [2,1]

Input: [1, 3, 5, 4, 1]
Output: [1, 4, 1, 3, 5]


Input: [1, 3, 2]
Output: [2, 1, 3]

Below will be the strategy

• Start from the left and find the first number which is less than the number on its right. Let’s say that number is found at index “first”

• Then find the smallest greater than the number on the index first on the right part of the array after the first index. Then replace that number with the number on the index first

• Sort the right part of the array after the index first

Program

Below is the program for the same

package main

import (
	"fmt"
	"sort"
)

func main() {
	nextPermutation([]int{1, 2, 3})

	nextPermutation([]int{2, 1})

	nextPermutation([]int{1, 3, 5, 4, 1})

	nextPermutation([]int{1, 3, 2})
}

func nextPermutation(nums []int) {

	numsLen := len(nums)
	first := -1
	second := -1

	for i, j := numsLen-2, numsLen-1; i >= 0; {
		if nums[i] < nums[j] {
			first = i
			second = j
			break
		} else {
			i--
			j--
		}
	}

	if !(first == -1) {
		smallestGreaterIndex := second
		for i := second + 1; i < numsLen; i++ {
			if nums[i] > nums[first] && nums[i] < nums[smallestGreaterIndex] {
				smallestGreaterIndex = i
			}
		}
		nums[first], nums[smallestGreaterIndex] = nums[smallestGreaterIndex], nums[first]

		sort.Slice(nums[second:numsLen], func(i, j int) bool {
			return nums[second+i] < nums[second+j]
		})
	}

	fmt.Println(nums)

}

Output

[1 3 2]
[2 1]
[1 4 1 3 5]
[2 1 3]

Note: Check out our Golang Advanced Tutorial. The tutorials in this series are elaborative and we have tried to cover all concepts with examples. This tutorial is for those who are looking to gain expertise and a solid understanding of golang - Golang Advance Tutorial

Also if you are interested in understanding how all design patterns can be implemented in Golang. If yes, then this post is for you -All Design Patterns Golang