leetcode-中等题-222. 完全二叉树的节点个数

https://leetcode-cn.com/problems/count-complete-tree-nodes/
递归的题目，左右中的后序遍历思想。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int countNodes(TreeNode root) {
        if(root == null) return 0; //确认终止条件
        int leftNum = countNodes(root.left);   //统计左结点的个数
        int rightNum = countNodes(root.right);  //统计右节点的个数
        int count = leftNum + rightNum + 1; //所有节点加起来
        return count;
    }
}