Permutations

Question: Given a collection of distinct integers, return all possible permutations.

Example:

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

You may view the full question here.

Approach: Thanks to this brilliant article and the amazing graphic illustration of the operation, here is a simple implementation using recursion—

//Approach
//Runtime: 1ms
//Memory usage: 38.5MB
class Solution {
    private List<List<Integer>> result = new ArrayList<>();
    public List<List<Integer>> permute(int[] nums) {
        recursive(nums, 0);
        return result;
    }
    
    private void recursive(int[] n, int start){
        if(start==n.length-1){
            List<Integer> res = new ArrayList<>();
            for(int curr : n){
                res.add(curr);
            }
            result.add(res);
        }
            
        for(int i = start;i<n.length; i++){
            int[] newN = new int[n.length];
            for(int j=0; j<n.length; j++){
                newN[j] = n[j];
            }
            //Swap
            int temp = newN[start];
            newN[start] = newN[i];
            newN[i] = temp;
            recursive(newN, start+1);
        }
    }
}

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Cheers & Chao!