LeetCode 77. Combinations 解题报告

77. Combinations

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Question

Total Accepted: 67261  Total Submissions: 201946  Difficulty: Medium

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],
]

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       求组合数， dfs求解，搜索K的每一位。

public class Combinations {

	static List>	ll	= new ArrayList>();

	public static void main(String[] args) {
		System.out.println(combine(5, 2));
		System.out.println(combine(4, 2));
	}

	public static List> combine(int n, int k) {
		ArrayList list = new ArrayList();
		ll.clear();
		dfs(n, k, 0, 1, list);
		return ll;
	}

	private static void dfs(int n, int k, int level, int idx, ArrayList list) {
		if (level >= k) {
			ll.add((ArrayList) list.clone());
			return;
		}
		for (int i = idx; i <= n; i++) {
			list.add(i);
			dfs(n, k, level + 1, i + 1, list);
			list.remove(new Integer(i));
		}
	}
}