[leetcode]46. Permutations全排列(给定序列无重复元素)

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

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

 

题意：

打印全排列

 

Solution1: Backtracking

形式化的表示递归过程：permutations(nums[0...n-1]) = {取出一个数字} + permutations(nums[0...n-1] - 该数字)

 

code

/*
Time: O(n!)
Space: O(n!). We allocate space to keep N! paths.
*/


class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        // corner case
        if (nums == null || nums.length == 0) return result;
        List<Integer> path = new ArrayList<>();
        helper(nums, path, result);
        return result; 
    }
    
    private void helper(int[] nums, List<Integer> path, List<List<Integer>> result){
        // base case
        if(path.size() == nums.length){
            result.add(new ArrayList<>(path));
            return;
        }
        
        for(int i = 0; i< nums.length; i++){
            if(path.contains(nums[i])) continue;
            path.add(nums[i]);
            helper(nums, path, result);
            path.remove(path.size() - 1);
        }
    }
}