78. Subsets

题目：

Given a set of distinct integers, nums, return all possible subsets.

Note: The solution set must not contain duplicate subsets.

For example,
If nums = [1,2,3], a solution is:

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

链接：https://leetcode.com/problems/subsets/#/description

4/22/2017

算法班

提到了递归的三要素？

 1 public class Solution {
 2     public List<List<Integer>> subsets(int[] nums) {
 3         List<List<Integer>> ret = new ArrayList<>();
 4 
 5         if (nums == null) {
 6             return ret;
 7         }
 8         
 9         Arrays.sort(nums);
10         
11         helper(ret, new ArrayList<Integer>(), nums, 0);
12         
13         return ret;
14     }
15     
16     private void helper(List<List<Integer>> ret,
17                         ArrayList<Integer> subset,
18                         int[] nums,
19                         int startIndex) {
20         // return condition is implicit here, since subset ends when reach to last element of nums
21         ret.add(new ArrayList<Integer>(subset));
22         for (int i = startIndex; i < nums.length; i++) {
23             subset.add(nums[i]);
24             helper(ret, subset, nums, i + 1);
25             subset.remove(subset.size() - 1);
26         }
27     }    
28 }

有人的总结：

https://discuss.leetcode.com/topic/46159/a-general-approach-to-backtracking-questions-in-java-subsets-permutations-combination-sum-palindrome-partitioning

python大哥的几种方法，包括reduce(), itertools.combinations()

https://discuss.leetcode.com/topic/15819/short-and-clear-solutions

有iterative方法的，但是不如普通方法个人感觉

https://discuss.leetcode.com/topic/19110/c-recursive-iterative-bit-manipulation-solutions-with-explanations