题目：

题解：

func countNodes(root *TreeNode) int {if root == nil {return 0}level := 0for node := root; node.Left != nil; node = node.Left {level++}return sort.Search(1<<(level+1), func(k int) bool {if k <= 1<<level {return false}bits := 1 << (level - 1)node := rootfor node != nil && bits > 0 {if bits&k == 0 {node = node.Left} else {node = node.Right}bits >>= 1}return node == nil}) - 1
}