在最坏情况下，找到n个元素中第二小的元素需要n+lgn-2次比较

首先两两比较找到最大的元素，需要n-1次，即二叉树的非叶子节点的个数。之后次最大的一定在和最大的元素比较过的元素中，共有lgn-1个，即树的高度。故加起来就是n+lgn-2

#include<iostream>
using namespace std;
class Node
{
public:
    Node();
    Node(int d);
    Node*left;
    Node*right;
    int data;
};
Node::Node()
{
    right = left = NULL;
}
Node::Node(int d)
{
    data = d;
    right = left = NULL;
}
class BinaryTree
{
public:
    Node*root;
    void create_tree(Node**node,int len);
    int min(int a, int b);
    int search_second_small();
    BinaryTree();

};
BinaryTree::BinaryTree()
{
    root = new Node();
}
int BinaryTree::min(int a, int b)
{
    return a < b ? a : b;
}
void BinaryTree::create_tree(Node**node, int len)
{
    if (len < 2)
        return;
    if (len == 2)
    {
        root->left = node[0];
        root->right = node[1];
        root->data = min(node[0]->data,node[1]->data);
    }
    else
    {
        int new_len = (len % 2) ? (len / 2 + 1) : (len / 2);
        Node**new_node = new Node*[new_len];
        int mod = len % 2;

        for (int i = 0; i < new_len - mod; i++)
        {
            new_node[i] = new Node(min(node[2*i]->data,node[2*i+1]->data));
            new_node[i]->left = node[2 * i];
            new_node[i]->right = node[2 * i + 1];
        }
        if (mod)
           new_node[new_len - 1] = node[len-1];
        create_tree(new_node, new_len);
        delete[] new_node;
    }
}
int BinaryTree::search_second_small()
{
    Node*p = root;
    int second = 0x7fffffff;
    while (p->left&&p->right)
    {
        if (p->data == p->left->data)
        {
            if (second > p->right->data)
                second = p->right->data;
            p = p->left;
        }
        else
        {
            if (second > p->left->data)
                second = p->left->data;
            p = p->right;
        }
    }
    return second;

}
int main()
{
    int arr[] = {1,3,2,4,6,0,9,8};
    int N = sizeof arr / sizeof arr[0];
    Node**node = new Node*[N];

    for (int i = 0; i < N; i++)
        node[i] = new Node(arr[i]);
    BinaryTree* bi_tree = new BinaryTree();
    bi_tree->create_tree(node, N);
    cout << bi_tree->root->data << endl;
    cout << bi_tree->search_second_small() << endl;
    return 0;

}