LeetCode——Unique Binary Search Trees II

Given an integer n, generate all structurally unique BST’s (binary search trees) that store values 1 … n.
Example:
Input: 3
Output:
[
[1,null,3,2],
[3,2,null,1],
[3,1,null,null,2],
[2,1,3],
[1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST’s shown below:

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

解法

循环选取每一个值作为根节点，然后将其所有可能的左子树与右子树互相组合

List<TreeNode> generateTrees(int n) {
        if(n<1)
        	return new ArrayList<TreeNode>();
		return core(1,n);
    }

	private List<TreeNode> core(int start, int end) {
		// TODO Auto-generated method stub
		List<TreeNode> res =new ArrayList<TreeNode>();
		if(start>end){
			res.add(null);
			return res;
		}
		for(int i=start;i<=end;i++)
		{
			List<TreeNode> left=core(start,i-1);
			List<TreeNode> right=core(i+1,end);
			for(TreeNode l:left){
				for(TreeNode r:right){
					TreeNode root=new TreeNode(i);
					root.left=l;
					root.right=r;
					res.add(root);
				}
			}
		}
		return res;
	}

Runtime: 2 ms, faster than 63.89% of Java online submissions for Unique Binary Search Trees II.
Memory Usage: 39.2 MB, less than 32.16% of Java online submissions for Unique Binary Search Trees II.