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;
}

}