題目：
Given a complete binary tree, count the number of nodes.

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 2h nodes inclusive at the last level h.

解答：

public class Solution {//學(xué)了enum的方法哈哈public enum Direction {LEFT, RIGHT}public int getDepth(TreeNode root, Direction dir) {int depth = 0;//這里是從root開始算起，所以求出來的結(jié)果是實際depth + 1while (root != null) {depth++;if (dir == Direction.LEFT) {root = root.left;} else {root = root.right;}}return depth;}public int countNodes(TreeNode root) {if (root == null) return 0;int leftDepth = getDepth(root, Direction.LEFT);int rightDepth = getDepth(root, Direction.RIGHT);if (leftDepth == rightDepth) {//1 << leftDepth是2^(leftDepth)return (1 << leftDepth) - 1;} else {//這一步很巧妙，把根結(jié)點加上，然后分治左結(jié)點和右結(jié)點，如果是葉子結(jié)點，在countNodes中就返回1return 1 + countNodes(root.left) + countNodes(root.right);}} }

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