LeetCode: 78. Subsets

题目描述

Given a set of distinct integers, nums, return all possible subsets (the power set).
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],
  []
]

题目大意：给定一个集合，求出它的所有子集。

解题思路

依次递归求解 k 个元素的子集。

AC 代码

class Solution {
public:
    // 在 [start, end) 区间查找 k 个元素的子集
    bool findSubsetsWithKElems(int start, int end, int cnt, 
                               vector<int>& record, int k, 
                               vector<vector<int>>& s, vector<int>& nums)
    {
        if (cnt == k)
        {
            s.push_back(record);
            return true;
        }
        else if (cnt > k || start > end)
        {
            return false;
        }

        for(int i = start; i < end; ++i)
        {
            ++cnt;
            record.push_back(nums[i]);
            findSubsetsWithKElems(i+1, end, cnt, record, k, s, nums);
            --cnt;
            record.pop_back();
        }

        return true;
    }
    vector<vector<int>> subsets(vector<int>& nums) {
        vector<vector<int>> s;
        vector<int> record;
        for(int i = 0; i <= nums.size(); ++i)
        {
            findSubsetsWithKElems(0, nums.size(), 0, record, i, s, nums);
        }

        return s;
    }
};