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.
解法一：递归，如果左侧高度等于右侧高度，则有2^h - 1个叶子节点。否则递归左右子树找节点个数。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int countNodes(TreeNode root) {
if (root == null) {
return 0;
}
int left = 0;
int right = 0;
TreeNode tmp = root;
while(tmp != null) {
left++;
tmp = tmp.left;
}
tmp = root;
while(tmp != null) {
right++;
tmp = tmp.right;
}
if (left == right) {
return (int)Math.pow(2, left) - 1;
}
return 1 + countNodes(root.left) + countNodes(root.right);
}
}
222. Count Complete Tree Nodes
原文：http://www.cnblogs.com/shini/p/4562534.html