【LeetCode】77. Combinations（C++）

地址：https://leetcode.com/problems/combinations/

题目：

Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

Example:

理解：

很像前面的permutation还有combinatio sum的问题，用backtracking解决。

实现：

感觉自己仿佛失了智，多用了一个nums数组，然而begin就可以代替了。

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<int> nums(n);
        for(int i=0;i<n;++i)
            nums[i]=i+1;
        vector<vector<int>> res;
        vector<int> tmp;
        backtracking(nums,k,res,tmp,0);
        return res;
    }
    
    void backtracking(vector<int>& nums,int k,vector<vector<int>>& res, vector<int>& tmp, int begin){
        if(tmp.size()==k){
            res.push_back(tmp);
            return;
        }
        for(int i=begin;i<nums.size();++i){
            tmp.push_back(nums[i]);
            backtracking(nums,k,res,tmp,i+1);
            tmp.pop_back();
        }
    }
};

修改一下：

class Solution {
public:
	vector> combine(int n, int k) {
		vector> res;
		vector tmp;
		backtracking(res, tmp, n, k, 1);
		return res;
	}
private:
	void backtracking(vector>& res, vector& tmp, int n, int k, int begin) {
		if (tmp.size() == k) {
			res.push_back(tmp);
			return;
		}
		for (int i = begin; i <= n; ++i) {
			tmp.push_back(i);
			backtracking(res, tmp, n, k, i + 1);
			tmp.pop_back();
		}
	}
};

上面第二种实现先后跑出了76ms和48ms?