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：提示说backtracking问题. 后来参考网络上的答案,感觉很棒，在此记录
public List<List<Integer>> combine(int n, int k) {
List<List<Integer>> result = new ArrayList<>();
if (n <= 0 || n < k) {
return result;
}
List<Integer> nodes = new ArrayList<>();
helper(n, k, 1, nodes, result);
return result;
}
private void helper(int n, int k, int start, List<Integer> nodes, List<List<Integer>> result) {
if(nodes.size()==k) {
result.add(new ArrayList<>(nodes));
return ;
}
for(int i=start;i<=n;i++) {
nodes.add(i);
helper(n, k, i+1, nodes, result);
nodes.remove(nodes.size()-1);
}
}