题目：

题解：

func countNodes(root *TreeNode) int {
    if root == nil {
        return 0
    }
    level := 0
    for 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 := root
        for node != nil && bits > 0 {
            if bits&k == 0 {
                node = node.Left
            } else {
                node = node.Right
            }
            bits >>= 1
        }
        return node == nil
    }) - 1
}