LeetCode 46. Permutations

Description

https://leetcode.com/problems/permutations/

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.

Explanation

Use backtracking to find all permutations. If the element has been visited, we can skip searching next permutations.

Python Solution

class Solution:
    def permute(self, nums: List[int]) -> List[List[int]]:
        results = []
        
        visited = set()
        self.helper(results, nums, [], 0, visited)
        
        return results
    
    def helper(self, results, nums, combination, start, visited):
        if len(combination) == len(nums):
            results.append(list(combination))
            return
        
        
        for i in range(len(nums)):
            if nums[i] in visited:
                continue
            
            visited.add(nums[i])
            combination.append(nums[i])
            self.helper(results, nums, combination, i + 1, visited)
            combination.pop()            
            visited.remove(nums[i])
            
  • Time complexity: O(2^N).
  • Space complexity: O(N).

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