46 Permutations

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

Given a collection ofdistinctnumbers, return all possible permutations.

For example,
[1,2,3]have the following permutations:

[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

分析及代码

因为permutation每一次都要从头走，所以需要标记哪些元素已经用过了

    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if(nums.length == 0) {
            return res;
        }
        boolean[] used = new boolean[nums.length];
        generate(res, new ArrayList<>(), nums, used);
        return res;
    }

    private void generate(List<List<Integer>> res, List<Integer> item, int[] nums, boolean[] used) {
        if(item.size() == nums.length) {
            res.add(new ArrayList<>(item));
            return;
        }
        for(int i = 0; i < nums.length; i++) {
            if(!used[i]) {
                used[i] = true;
                item.add(nums[i]);
                generate(res, item, nums, used);
                used[i] = false;
                item.remove(item.size() - 1);
            }
        }
    }

O(n!)