思路：
    通法就是递归，其他方法暂不考虑

    1. /**
    2.  * Definition for a binary tree node.
    3.  * function TreeNode(val, left, right) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.left = (left===undefined ? null : left)
    6.  *     this.right = (right===undefined ? null : right)
    7.  * }
    8.  */
    9. /**
    10.  * @param {TreeNode} root
    11.  * @return {number}
    12.  */
    13. var countNodes = function(root) {
    14.     let result = 0
    15.     let nodes = data => {
    16.         if (data) {            
    17.             nodes(data.left)
    18.             nodes(data.right)
    19.             result++
    20.         }
    21.     }
    22.     nodes(root)
    23.     return result
    24. };

    image.png