Next permutation in python gfg

Given an array arr[] of size N, the task is to print the lexicographically next greater permutation of the given array. If there does not exist any greater permutation, then print the lexicographically smallest permutation of the given array.

Examples:

Input: N = 6, arr = {1, 2, 3, 6, 5, 4}
Output: {1, 2, 4, 3, 5, 6}
Explanation: The next permutation of the given array is {1, 2, 4, 3, 5, 6}.

Input: N = 3, arr = {3, 2, 1}
Output: {1, 2, 3}
Explanation: As arr[] is the last permutation. 
So, the next permutation is the lowest one.

Brute Force Approach :

A simple way to solve this problem is to generate all the permutations of the given array and return the permutation which is just greater than the given array. 

Time Complexity: O(N * N!), as the total possible permutations are N!
Auxiliary Space: O(N), for storing the permutation in some data structure.

Next Permutation in linear time complexity:

Illustration: 

Let’s try some examples to see if we can recognize some patterns. 

[3, 1, 3] = next greater number is 331
[5, 1, 3] = next greater number is 531
[1, 2, 3] = next greater number is 132
[1, 3, 5, 4] = next greater number is 1435
[3, 2, 1] = we can’t form a number greater than the current number from all the possible permutations

So, it is clear that to get the next permutation we will have to change the number in a position which is as right as possible. Each permutation (except the very first) has a increasing suffix. Now if we change the pattern from the pivot point (where the increasing suffix breaks) to its next possible lexicographic representation we will get the next greater permutation.

To understand how to change the pattern from pivot, see the below image:

Observation of Next permutation: 

Next permutation in python gfg

Illustration of next_permutation

Follow the steps below to implement the above observation:

  • Iterate over the given array from end and find the first index (pivot) which doesn’t follow property of non-increasing suffix, (i.e,  arr[i] < arr[i + 1]).
  • Check if pivot index does not exist 
    • This means that the given sequence in the array is the largest as possible. So, swap the complete array.
  • Otherwise, Iterate the array from the end and find for the successor of pivot in suffix.
  • Swap the pivot and successor
  • Minimize the suffix part by reversing the array from pivot + 1 till N.

Below is the implementation of the above approach:

C++

#include <bits/stdc++.h>

using namespace std;

void nextPermutation(vector<int>& arr)

{

    int n = arr.size(), i, j;

    for (i = n - 2; i >= 0; i--) {

        if (arr[i] < arr[i + 1]) {

            break;

        }

    }

    if (i < 0) {

        reverse(arr.begin(), arr.end());

    }

    else {

        for (j = n - 1; j > i; j--) {

            if (arr[j] > arr[i]) {

                break;

            }

        }

        swap(arr[i], arr[j]);

        reverse(arr.begin() + i + 1, arr.end());

    }

}

int main()

{

    vector<int> arr = { 1, 2, 3, 6, 5, 4 };

    nextPermutation(arr);

    for (auto i : arr) {

        cout << i << " ";

    }

    return 0;

}

Time Complexity: O(N), where N is the size of the given array.
Auxiliary Space: O(1)


How do you get the next permutation in Python?

Suppose we want to implement the next permutation method, that method rearranges numbers into the lexicographically next greater permutation of numbers. If such arrangement is not possible, this method will rearrange it as the lowest possible order (That is actually, sorted in ascending order).

How do you find the next permutation of the GFG?

For example, lexicographically next permutation of “gfg” is “ggf” and the next permutation of “acb” is “bac”..
In the worst case, the first step of next_permutation takes O(n) time..
The binary search takes O(log n) time..
The reverse takes O(n) time..

How do you find the next permutation?

14 Answers.
Find the highest index i such that s[i] < s[i+1] . If no such index exists, the permutation is the last permutation..
Find the highest index j > i such that s[j] > s[i] . ... .
Swap s[i] with s[j] ..
Reverse the order of all of the elements after index i till the last element..

What is next permutation?

The lexicographically next permutation is basically the greater permutation. For example, the next of “ACB” will be “BAC”. In some cases, the lexicographically next permutation is not present, like “BBB” or “DCBA” etc. In C++ we can do it by using a library function called next_permutation().

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