77. Combinations

难度: Medium

题目描述

Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].

You may return the answer in any order.

 

Example 1:

Input: n = 4, k = 2
Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
Explanation: There are 4 choose 2 = 6 total combinations.
Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.

Example 2:

Input: n = 1, k = 1
Output: [[1]]
Explanation: There is 1 choose 1 = 1 total combination.

 

Constraints:

• 1 <= n <= 20
• 1 <= k <= n

解题思路

代码实现

解决方案

java

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> data = new ArrayList<>();
        backStack(data, new ArrayList(), n, k, 1);
        return data;
    }

    private void backStack(List<List<Integer>> data, List<Integer> current, int n, int k, int start) {
        if (current.size() == k) {
            data.add(new ArrayList<>(current));
            return;
        }
        for (int i = start; i <= n; i++) {
            current.add(i);
            backStack(data, current, n, k, i + 1);
            current.remove(current.size() - 1);
        }
    }
}