leetcode解题: Combinations (77)

发表于 | 分类于

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

For example,
If n = 4 and k = 2, a solution is:

1
2
3
4
5
6
7
8
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

解法1： Backtracking

常规的backtracking的题目，这里的特殊点是每一次要更新还剩下需要挑选的数字的个数。 然后用另外一个变量pos记录当前已经探索到的数字的位置。

C++

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        
        vector<vector<int>> res;
        if (k <= 0 || n == 0) {
            return res;
        }
        
        vector<int> cur;
        
        helper(n, k, 1, cur, res);
        return res;
        
    }
    
    void helper(int n, int k, int pos, vector<int>& cur, vector<vector<int>>& res) {
        
        if (k == 0) {
            res.push_back(cur);
            return;
        }    
        
        for (int i = pos; i <= n; ++i) {
            cur.push_back(i);
            helper(n, k - 1, i + 1, cur, res);
            cur.pop_back();
        }
        
        return;
        
    }
    
};

Java

1