Segmentation fault on recursion function c++ - c++

Question

I was working on Pre-order traverse Binary Tree algorithm. But I meet with Segmentation fault:11 error. The codes are shown as follow.
I want to know why this error happens. By the way, I have tried non-recursion algorithm preorder()，segmentation fault still happen.
The environment:
macOS, clang-800.0.38
struct Node{
char val;
Node* left;
Node* right;
};
void preorder(Node *T){
if(T!=NULL){
cout << T->val;
preorder(T->left);
preorder(T->right);
}
}
int main(){
Node *T = (Node *)malloc(sizeof(Node));
Node *p = T;
p->val = 'A';
p->left = (Node *)malloc(sizeof(Node));
p->left->val = 'B';
p->right = (Node *)malloc(sizeof(Node));
p->right->val = 'C';
preorder(T);
return 0;
}

Answer 1

You need to initialize left and right member variables of your nodes to null pointers.
Anyway, if you use C++, use C++ and not C constructs. This is an exemplary C++14 version of your code:
#include <iostream>
#include <memory>
struct Node{
Node(char a) : val(a) { };
char val;
std::unique_ptr<Node> left, right;
};
void preorder(Node* p) {
if (p) {
std::cout << p->val;
preorder(p->left.get());
preorder(p->right.get());
}
}
int main() {
auto root = std::make_unique<Node>('A');
root->left = std::make_unique<Node>('B');
root->right = std::make_unique<Node>('C');
preorder(root.get());
}

Answer 2

The problem is left and right of your child nodes are not initialized to NULL. Due to previous value stored in that memory location the program is encountering non-null value in your if(T!=nullptr) statement and executing the if block.
struct Node{
char val;
Node* left;
Node* right;
};
void preorder(Node *T){
if(T!=nullptr){
cout << T->val;
preorder(T->left);
preorder(T->right);
}
}
int main(){
Node *T = (Node *)malloc(sizeof(Node));
Node *p = T;
p->val = 'A';
p->left = (Node *)malloc(sizeof(Node));
p->left->val = 'B';
p->left->left = nullptr; //Initialize to NULL
p->left->right = nullptr; //Initialize to NULL
p->right = (Node *)malloc(sizeof(Node));
p->right->val = 'C';
p->right->left = nullptr; //Initialize to NULL
p->right->right = nullptr; //Initialize to NULL
preorder(T);
return 0;
}