题目： 

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两个方法本质就是决策树的画法不同

方法一解析： 

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代码： 

class Solution {private List<List<Integer>> ret;//返回结果private List<Integer> path;//记录路径，注意返回现场public List<List<Integer>> subsets(int[] nums) {path = new ArrayList<>();ret = new ArrayList<>();dfs(nums,0);return ret;}//i表示选择到了那一层private void dfs(int[] nums, int indx){//递归出口if(indx == nums.length){ret.add(new ArrayList<>(path));return;}//选某个元素path.add(nums[indx]);dfs(nums,indx+1);path.remove(path.size()-1);//回复现场//不选某个元素dfs(nums,indx+1);}
}

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方法二解析：  

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代码： 

 private List<List<Integer>> ret;//返回结果private List<Integer> path;//记录路径public List<List<Integer>> subsets(int[] nums) {path = new ArrayList<>();ret = new ArrayList<>();dfs(nums,0);return ret;}private void dfs(int[] nums, int pos){ret.add(new ArrayList<>(path));for(int i = pos; i < nums.length; i++){path.add(nums[i]);//每一层只能往选过的元素，再往选dfs(nums,i+1);path.remove(path.size()-1);//回复现场}}