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？

public class Solution {
    public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> ret = new ArrayList<>();

        if (nums == null) {
            return ret;
        }
        
        Arrays.sort(nums);
        
        helper(ret, new ArrayList<Integer>(), nums, 0);
        
        return ret;
    }
    
    private void helper(List<List<Integer>> ret,
                        ArrayList<Integer> subset,
                        int[] nums,
                        int startIndex) {
        // return condition is implicit here, since subset ends when reach to last element of nums
        ret.add(new ArrayList<Integer>(subset));
        for (int i = startIndex; i < nums.length; i++) {
            subset.add(nums[i]);
            helper(ret, subset, nums, i + 1);
            subset.remove(subset.size() - 1);
        }
    }    
}

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

更多讨论：

https://discuss.leetcode.com/category/86/subsets