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Java-based LeetCode algorithm problem solutions, regularly updated

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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:

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

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

    private void permuteHelper(int[] nums, List current, List> 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.