LeetCode 77. Combinations

77. Combinations

Difficulty: Medium

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

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

Solution

Language: Java

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new ArrayList<>();
        if (n <= 0 || k <= 0) {
            return result;
        }
        
        helper(1, n, k, result, new ArrayList<>(k));
        return result;
    }
    
    private void helper(int cur, int n, int k, List<List<Integer>> result, List<Integer> one) {
        if (k == 0) {
            result.add(new ArrayList<>(one));
            return;
        }
        for (int i = cur; i <= n; i++) {
            one.add(i);
            helper(i + 1, n, k - 1, result, one);
            one.remove(one.size() - 1);
        }
    }
}

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new ArrayList<>();
        if (n <= 0 || k <= 0) {
            return result;
        }
        
        helper(1, n, k, result, new int[k], 0);
        return result;
    }
    
    private void helper(int cur, int n, int k, List<List<Integer>> result, int[] one, int index) {
        if (k == 0) {
            List<Integer> tmp = new ArrayList<>(k);
            for (int a : one) {
                tmp.add(a);
            }
            result.add(tmp);
            return;
        }
        for (int i = cur; i <= n; i++) {
            one[index++] = i;
            helper(i + 1, n, k - 1, result, one, index);
            index--;
        }
    }
}