Programming
Leetcode-77. Combinations
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.
<解題>
class Solution {
public List<List<Integer>> combine(int n, int k) {
int[] nums = new int[n];
for(int i =0; i<n; i++){
nums[i] = i+1;
}
List<List<Integer>> res = new ArrayList<>();
helper(res, nums, new ArrayList<>(), 0, k);
return res;
}
public void helper (List<List<Integer>> res, int[] nums, List<Integer> temp, int index, int count){
if(count == 0){
res.add(new ArrayList<>(temp));
return;
}
for(int i = index ; i < nums.length; i++){
temp.add(nums[i]);
helper(res, nums, temp, i + 1, count - 1);
temp.remove(temp.size() -1);
}
}
}
Time: O(C(n, k)) = n! / (k! * (n - k)!) Space: O(C(n, k) + k)