Java for LeetCode 077 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],
]

解题思路：

DFS，JAVA实现如下：

	public static void main(String args[]) {
		List<List<Integer>> list = combine(5, 2);
		System.out.println(list.size());
		for (List<Integer> alist : list)
			System.out.println(alist + "*");
	}

	static public List<List<Integer>> combine(int n, int k) {
		List<List<Integer>> list = new ArrayList<List<Integer>>();
		if (n <= 0 || k > n)
			return list;
		dfs(list, n, k, 0);
		return list;
	}

	static List<Integer> alist = new ArrayList<Integer>();

	static void dfs(List<List<Integer>> list, int n, int k, int depth) {
		if (depth >= k) {
			list.add(new ArrayList<Integer>(alist));
			return;
		}
		if (depth == 0)
			for (int i = 1; i <= n - k + 1; i++) {
				alist.add(i);
				dfs(list, n, k, depth + 1);
				alist.remove(alist.size() - 1);
			}
		else
			for (int i = 1; i <= n - alist.get(alist.size() - 1) + k - depth - 1; i++) {
				alist.add(alist.get(alist.size() - 1) + i);
				dfs(list, n, k, depth + 1);
				alist.remove(alist.size() - 1);
			}
	}