【LeetCode】98. Combinations

题目描述(Medium)

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

题目链接

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

Example 1:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

算法分析

方法一：dfs，类似【LeetCode】96. Permutations

提交代码(方法一)：

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> result;
        vector<int> path;
        dfs(path, 1, n, k, result);
        
        return result;
    }
    
    void dfs(vector<int>& path, int step, int n, int k, 
             vector<vector<int>>& result) {
        if (path.size() == k)
        {
            result.push_back(path);
            return;
        }
        
        for (int i = step; i <= n; ++i)
        {
            path.push_back(i);
            dfs(path, i + 1, n, k, result);
            path.pop_back();
        }
    }    
};