【leetcode】Count Complete Tree Nodes (Java)

Count Complete Tree Nodes

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 2hnodes inclusive at the last level h.
题意就是求完全二叉树的节点个数。

解题思路：

这道题比较简单，要利用完全二叉树的性质简化过程：即如果当树的左右节点的高度是相同时，就是一个满二叉树，就可以得到他的节点数为（2^height - 1），其他的情况就可以递归了。

/**
* 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 rightLevel = countRightLevel(root);
int leftLevel = countLeftLevel(root);

if(rightLevel == leftLevel)
return (1 << leftLevel) - 1;
return countNodes(root.left) + 1 + countNodes(root.right);
}
public int countRightLevel(TreeNode root){
int level = 0;
TreeNode temp = root;
while(temp != null){
level++;
temp = temp.right;
}
return level;
}
public int countLeftLevel(TreeNode root){
int level = 0;
TreeNode temp = root;
while(temp != null){
level++;
temp = temp.left;
}
return level;
}
}