/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func countNodes(root *TreeNode) (cnt int) {
	if root == nil {
		return
	}
	lvs := []*TreeNode{root}
	for n := len(lvs); n != 0; n = len(lvs) {
		node := lvs[n-1]
		lvs = lvs[:n-1]
		cnt++
		if node.Left != nil {
			lvs = append(lvs, node.Left)
		}
		if node.Right != nil {
			lvs = append(lvs, node.Right)
		}
	}
	return
}