1115 Counting Nodes in a BST(禁止输出容器map[不存在的数])

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

大致题意就是给出一个序列，构造一棵二叉查找树，输出最大深度和次最大深度的结点个数之和。

方法一，BFS

 1 #include<iostream>
 2 #include<unordered_map>
 3 #include<queue>
 4 #include<algorithm>
 5 using namespace std;
 6 
 7 struct Node {
 8     int data,level;
 9     Node* lchild;
10     Node* rchild;
11 };
12 
13 void insert(Node *&root,int data) {
14     if(root == NULL) {
15         root = new Node;
16         root->data = data;
17         root->lchild = root->rchild = NULL;
18     } else if(data <= root->data) insert(root->lchild,data); //此题BST定义
19     else insert(root->rchild,data);
20 }
21 
22 unordered_map<int,int> mp;
23 int maxlevel = -1;
24 void BFS(Node* root) {
25     queue<Node*> q;
26     root->level = 1;
27     q.push(root);
28     while(!q.empty()) {
29         Node* node = q.front();
30         mp[node->level]++;
31         maxlevel = max(maxlevel,node->level);
32         q.pop();
33         if(node->lchild != NULL) {
34             node->lchild->level = node->level+1;
35             q.push(node->lchild);
36         }
37         if(node->rchild != NULL) {
38             node->rchild->level = node->level+1;
39             q.push(node->rchild);
40         }
41     }
42 }
43 int main() {
44     int n,t;
45     cin>>n;
46     Node* root = NULL;
47     for(int i = 0; i < n; ++i) {
48         cin>>t;
49         insert(root,t);
50     }
51     BFS(root);
52     printf("%d + %d = %d",mp[maxlevel],mp[maxlevel-1],mp[maxlevel]+mp[maxlevel-1]);
53     return 0;
54 }

方法二，DFS

 1 #include<iostream>
 2 #include<unordered_map>
 3 #include<queue>
 4 #include<algorithm>
 5 using namespace std;
 6 
 7 struct Node {
 8     int data,level;
 9     Node* lchild;
10     Node* rchild;
11 };
12 
13 void insert(Node *&root,int data) {
14     if(root == NULL) {
15         root = new Node;
16         root->data = data;
17         root->lchild = root->rchild = NULL;
18     } else if(data <= root->data) insert(root->lchild,data); //此题BST定义
19     else insert(root->rchild,data);
20 }
21 
22 unordered_map<int,int> mp;
23 int maxlevel = -1;
24 void DFS(Node* root,int level) {
25     if(root == NULL) {
26         maxlevel = max(maxlevel,level-1);
27         return;
28     }
29     mp[level]++;
30     DFS(root->lchild,level+1);
31     DFS(root->rchild,level+1);
32 }
33 int main() {
34     int n,t;
35     cin>>n;
36     Node* root = NULL;
37     for(int i = 0; i < n; ++i) {
38         cin>>t;
39         insert(root,t);
40     }
41     DFS(root,1);
42     printf("%d + %d = %d",mp[maxlevel],mp[maxlevel-1],mp[maxlevel]+mp[maxlevel-1]);
43     return 0;
44 }