LeetCode 77: Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

public class Solution {
    private List<List<Integer>> result;
    private int[] x;
    private int nN;
    private int nK;
    
    public List<List<Integer>> combine(int n, int k) {
        // Backtracking problem
        result = new LinkedList<>();
        
        if (n<=0 || k<=0 || k>n)
            return result;
        
        nN = n;
        nK = k;
        x = new int[k];
        
        for (int i = 1; i <= n-k+1; i++)
        {
            x[0] = i;
            backTracking(i, 1);
        }
        
        return result;
    }
    
    private void backTracking(int i, int cnt) // cnt -- counting times
    {
        if (cnt == nK)
        {
            List<Integer> ret = new LinkedList<>();
            
            for (int num : x)
                ret.add(num);
                
            result.add(ret);
        }
        else
        {
            for (int j = i+1; j <= nN; j++)
            {
                x[cnt] = j;
                
                if (nN-(j+1) >= (nK-1)-(cnt+1)) // The left numbers is enough
                    backTracking(j, cnt+1);
            }
        }
    }
}