【Leetcode】Count Complete Tree Nodes

题目链接：https://leetcode.com/problems/count-complete-tree-nodes/

题目：

complete

Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2hnodes inclusive at the last level h.

思路：

满二叉树每层结点数目是2^(h-1)，共2^h-1。利用上述定理，递归求结点个数。还有求2^h-1用移位不会超时，用math.pow会超时。

算法：

public int countNodes(TreeNode root) {
		if (root == null)
			return 0;
		//求root左右高度
		TreeNode tmp = root.left;
		int leftHeight = 1;
		while (tmp != null) {
			tmp = tmp.left;
			leftHeight++;
		}
		tmp = root.right;
		int rightHeight = 1;
		while (tmp != null) {
			tmp = tmp.right;
			rightHeight++;
		}
		//====end
		if (leftHeight == rightHeight)
			return (2 << (leftHeight - 1)) - 1; //如果用Math.pow计算会超时，so why?
		return 1 + countNodes(root.left) + countNodes(root.right);
	}