lintcode-medium-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]]

思路还是一样的，回溯算法，只是判断条件稍微不通而已

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 List<List<Integer>> combine(int n, int k) {
        // write your code here

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

        if(n < 1 || k > n)
            return result;

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

        helper(result, line, n, k, 1);

        return result;
    }

    public void helper(List<List<Integer>> result, List<Integer> line, int n, int k, int start){

        if(line.size() == k){
            result.add(new ArrayList<Integer>(line));
            return;
        }

        for(int i = start; i <= n; i++){
            line.add(i);
            helper(result, line, n, k, i + 1);
            line.remove(line.size() - 1);
        }

        return;
    }

}