Combinations 152

Question

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

Example

For example,

If n = 4 and k = 2, a solution is:

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

Solution

Comination模版。注意是从1开始到n。

代码如下：

public class Solution {
    /**
     * @param n: Given the range of numbers
     * @param k: Given the numbers of combinations
     * @return: All the combinations of k numbers out of 1..n
     */
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        // write your code here
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(n <= 0 || k <= 0 || n < k){
            return result;
        }

        ArrayList<Integer> list = new ArrayList<Integer>();
        combineHelper(result, list, n, k, 1);

        return result;
    }

    private void combineHelper(ArrayList<ArrayList<Integer>> result, ArrayList<Integer> list, int n, int k, int pos){
        if(list.size() == k){
            result.add(new ArrayList<Integer>(list));
            return;
        }

        for(int i = pos; i <= n; i++){
            list.add(i);
            combineHelper(result, list, n, k, i + 1);
            list.remove(list.size() - 1);
        }
    }
}