Given a collection of distinct numbers, return all possible permutations.
和subsets的区别在于这次是permutation, [1, 2, 3]和[2, 1, 3]都是合理的答案, 因此不再需要start pos. 但为了防止出现[1, 1, 1,]这样的答案, 还需要用一个set记录当前path已经访问过了谁.
class Solution {
private void helper(int[] nums, boolean[] visited, List<Integer> path, List<List<Integer>> res) {
if (path.size() == nums.length) {
res.add(new ArrayList<>(path));
}
for (int i = 0; i < nums.length; i++) {
if (visited[i]) {
continue;
}
visited[i] = true;
path.add(nums[i]);
helper(nums, visited, path, res);
visited[i] = false;
path.remove(path.size() - 1);
}
}
public List<List<Integer>> permute(int[] nums) {
List<List<Integer>> res = new ArrayList<>();
helper(nums, new boolean[nums.length], new ArrayList<>(), res);
return res;
}
}
时间复杂度O(N!). 指数级. 空间O(N).