https://leetcode.com/problems/combinations/description/
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
🚡java
//backtrack
class Solution {
private int n;
private int k;
public List<List<Integer>> combine(int n, int k) {
List<List<Integer>> list = new ArrayList<>();
List<Integer> comb = new ArrayList<>();
this.n = n;
this.k = k;
backtrack(1, list, comb);
return list;
}
private void backtrack(int firstNum, List<List<Integer>> list, List<Integer> comb){
if(comb.size() == k){
list.add(new ArrayList<>(comb)); // store a deep copy
return;
}
for (int num = firstNum; num <= n; num++) {
// 1. Include the current number in the combination
comb.add(num);
// 2. Recursively build combinations with numbers greater than the current one
// This ensures we don't use the same number twice and maintains ascending order
backtrack(num + 1, list, comb);
// 3. Backtrack: Remove the last number to try other possibilities
// (Undo the choice to explore other paths)
comb.remove(comb.size() - 1);
}
return;
}
}