题解

Medium | Backtracking

class Solution {
public:
    vector<vector<int>> subsets(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        vector<vector<int>> sols;
        vector<int> sol;
        for(int i = 0; i <= nums.size(); i++) {
            helper(nums, 0, i, sol, sols);
        }
        return sols;
    }
    
    void helper(vector<int>& nums, int pos, int k, vector<int>& sol, vector<vector<int>>& sols) {
        if(k == sol.size()) {
            sols.push_back(sol);
            return;
        }
        
        for(int i = pos; i < nums.size(); i++) {
            sol.push_back(nums[i]);
            helper(nums, i+1, k, sol, sols);
            sol.pop_back();
        }
    }
};

其实过程有一点冗余，可以简化为：

class Solution {
public:
    vector<vector<int>> subsets(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        vector<vector<int>> sols;
        vector<int> sol;
        helper(nums, 0, sol, sols);
        return sols;
    }
    
    void helper(vector<int>& nums, int pos, vector<int>& sol, vector<vector<int>>& sols) {
        sols.push_back(sol);
        
        for(int i = pos; i < nums.size(); i++) {
            sol.push_back(nums[i]);
            helper(nums, i+1, sol, sols);
            sol.pop_back();
        }
    }
};