LeetCode-in-All

46. Permutations

Medium

Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order.

Example 1:

Input: nums = [1,2,3]

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

Example 2:

Input: nums = [0,1]

Output: [[0,1],[1,0]]

Example 3:

Input: nums = [1]

Output: [[1]]

Constraints:

• 1 <= nums.length <= 6
• -10 <= nums[i] <= 10
• All the integers of nums are unique.

To solve the “Permutations” problem in Java with a Solution class, we can follow these steps:

1. Define a Solution class.
2. Define a method named permute that takes an array of distinct integers nums as input and returns a list of all possible permutations.
3. Create an empty list to store the result permutations.
4. Call a recursive helper function named permuteHelper to generate permutations.
5. Inside the permuteHelper function:
   • If the current permutation size equals the length of the input array nums, add a copy of the current permutation to the result list.
   • Otherwise, iterate through each element of nums:
     • If the current element is not already in the permutation, add it to the current permutation, and recursively call permuteHelper with the updated permutation and the remaining elements of nums.
     • After the recursive call, remove the last element from the permutation to backtrack.
6. Return the result list.

Here’s the implementation:

import java.util.ArrayList;
import java.util.List;

public class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        permuteHelper(nums, new ArrayList<>(), result);
        return result;
    }

    private void permuteHelper(int[] nums, List<Integer> current, List<List<Integer>> result) {
        if (current.size() == nums.length) {
            result.add(new ArrayList<>(current));
            return;
        }

        for (int num : nums) {
            if (!current.contains(num)) {
                current.add(num);
                permuteHelper(nums, current, result);
                current.remove(current.size() - 1);
            }
        }
    }
}

This implementation provides a solution to the “Permutations” problem in Java. It generates all possible permutations of the given array of distinct integers using backtracking.

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