https://pintia.cn/problem-sets/994805342720868352/problems/994805355987451904

A Binary Search Tree (BST) is recursively defined as a binary tree which has the following properties:

• The left subtree of a node contains only nodes with keys less than or equal to the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

Insert a sequence of numbers into an initially empty binary search tree. Then you are supposed to count the total number of nodes in the lowest 2 levels of the resulting tree.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (≤) which is the size of the input sequence. Then given in the next line are the N integers in [ which are supposed to be inserted into an initially empty binary search tree.

Output Specification:

For each case, print in one line the numbers of nodes in the lowest 2 levels of the resulting tree in the format:

n1 + n2 = n

where n1 is the number of nodes in the lowest level, n2 is that of the level above, and n is the sum.

Sample Input:

9

25 30 42 16 20 20 35 -5 28

Sample Output:

2 + 4 = 6

代码：

#include <bits/stdc++.h>

using namespace std;

int N;

vector<int> v(1000);

int depth = -1;

struct Node {

    int val;

    struct Node *left, *right;

};

Node* BuildBST(Node *root, int x) {

    if(!root) {

        root = new Node();

        root -> val = x;

        root -> left = NULL;

        root -> right = NULL;

    } else if(x <= root -> val)

        root -> left = BuildBST(root -> left, x);

    else root -> right = BuildBST(root -> right, x);

    return root;

}

void dfs(Node* root, int step) {

    if(!root) {

        depth = max(depth, step);

        return ;

    }

    v[step] ++;

    dfs(root -> left, step + 1);

    dfs(root -> right, step + 1);

}

int main() {

    scanf("%d", &N);

    Node *root = NULL;

    for(int i = 0; i < N; i ++) {

        int num;

        scanf("%d", &num);

        root = BuildBST(root, num);

    }

    dfs(root, 0);

    printf("%d + %d = %d\n", v[depth - 1], v[depth - 2], v[depth - 1] + v[depth - 2]);

    return 0;

}

　　先建树然后 dfs 记录每一层的节点数目存在数组里 和上个提交的题目比较类似了