[LeetCode 77] Combinations

题目链接：combinations

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import java.util.ArrayList;
import java.util.List;

/**
 * 
		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],
		]
 *
 */

public class Combinations {

//	26 / 26 test cases passed.
//	Status: Accepted
//	Runtime: 262 ms
//	Submitted: 0 minutes ago

	//时间复杂度O(n!) 空间复杂度O(n)
	public List> conbinations = new ArrayList>();
    public List> combine(int n, int k) {
        
    	combine(n, 1, new ArrayList(), k);
    	return conbinations;
    }
    
    public void combine(int n, int begin, List conbination, int k) {
    	if(conbination.size() == k) {
    		conbinations.add(conbination);
    		return;
    	}
    	for (int i = begin; i <= n; i++) {
			List list = new ArrayList(conbination);
			list.add(i);
			combine(n, i + 1, list, k);
		}
    }
    
	public static void main(String[] args) {

	}

}