LeetCode 222. 完全二叉树的节点个数

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/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var countNodes = function (root) {
  if (!root) return 0;
  return 1 + countNodes(root.left) + countNodes(root.right);
};