题目：

题解：

bool exists(struct TreeNode* root, int level, int k) {
    int bits = 1 << (level - 1);
    struct TreeNode* node = root;
    while (node != NULL && bits > 0) {
        if (!(bits & k)) {
            node = node->left;
        } else {
            node = node->right;
        }
        bits >>= 1;
    }
    return node != NULL;
}

int countNodes(struct TreeNode* root) {
    if (root == NULL) {
        return 0;
    }
    int level = 0;
    struct TreeNode* node = root;
    while (node->left != NULL) {
        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;
}