Solved! Leetcode 77. Combinations

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

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

Solution

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

    private void findPermute(int start, int end, int k, List<Integer> curList) {
        if(curList.size() == k) {
            result.add(new ArrayList<>(curList));
            return;
        }

        for(int i=start;i<=end;i++) {
            curList.add(i);
            findPermute(i+1, end, k, curList);
            curList.remove(curList.lastIndexOf(i));
        }
    }
}

class Solution {

List<List<Integer>> result = new ArrayList<>();

public List<List<Integer>> combine(int n, int k) {

findPermute(1, n, k, new ArrayList<Integer>());

return result;

}

private void findPermute(int start, int end, int k, List<Integer> curList) {

if(curList.size() == k) {

result.add(new ArrayList<>(curList));

return;

}

for(int i=start;i<=end;i++) {

curList.add(i);

findPermute(i+1, end, k, curList);

curList.remove(curList.lastIndexOf(i));

}

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