题目：

题解：

class Solution {
public:int countNodes(TreeNode* root) {if (root == nullptr) {return 0;}int level = 0;TreeNode* node = root;while (node->left != nullptr) {level++;node = node->left;}int low = 1 << level, high = (1 << (level + 1)) - 1;while (low < high) {int mid = (high - low + 1) / 2 + low;if (exists(root, level, mid)) {low = mid;} else {high = mid - 1;}}return low;}bool exists(TreeNode* root, int level, int k) {int bits = 1 << (level - 1);TreeNode* node = root;while (node != nullptr && bits > 0) {if (!(bits & k)) {node = node->left;} else {node = node->right;}bits >>= 1;}return node != nullptr;}
};