LeetCode78：子集

递归，回溯法

public class Solution {
    List<List<Integer>> res = new ArrayList<>();
    List<Integer> path = new ArrayList<>();

    public List<List<Integer>> subsets(int[] nums) {
        dfs(nums, 0);
        return res;
    }

	// 前begin位已固定，从begin开始搜索
    private void dfs(int[] nums, int begin) {
        int l = nums.length;
        res.add(new ArrayList<>(path));
        for (int i = begin; i < l; i++) {
            path.add(nums[i]);
            dfs(nums, i + 1);
            path.remove(path.size() - 1);
        }
    }
}

非递归，位运算

public class Solution2 {
    List<Integer> t = new ArrayList<Integer>();
    List<List<Integer>> ans = new ArrayList<List<Integer>>();

    public List<List<Integer>> subsets(int[] nums) {
        int n = nums.length;
        for (int mask = 0; mask < (1 << n); ++mask) {
            t.clear();
            for (int i = 0; i < n; ++i) {
                if ((mask & (1 << i)) != 0) {
                    t.add(nums[i]);
                }
            }
            ans.add(new ArrayList<Integer>(t));
        }
        return ans;
    }
}