#include<stdio.h>
#include<stdlib.h>

typedef struct node
{
    int data;
    struct node * left;
    struct node * right;
} Node;

Node * createNode(int val);
Node * createTree();
void freeTree(Node * node);

void preOrder(Node * node);

// 先序创建二叉树
int main() {

    Node * root = createTree();
    preOrder(root);
    freeTree(root);
    return 0;
}

Node * createNode(int val) {
    Node * node = (Node *)malloc(sizeof(Node));
    if (node == NULL)
    {
        printf("内存申请异常");
        exit(-1);
    }
    node->data = val;
    node->left = NULL;
    node->right = NULL;
    return node;
}

Node * createTree() {
    int val = 0;
    printf("请输入创建树的数据 0为NULL:");
    scanf("%d", &val);
    if (val == 0)
    {
        return NULL;
    }

    Node * node = createNode(val);
    printf("请输入%d的左子节点:", val);
    node->left = createTree();
    printf("请输入%d的右子节点:", val);
    node->right = createTree();

    return node;
}

void preOrder(Node * node) {
    if (node == NULL)
    {
        return;
    }
    
    printf("%d ", node->data);
    preOrder(node->left);
    preOrder(node->right);
}

void freeTree(Node * root) {
    if (root == NULL)
    {
        return;
    }

    freeTree(root->left);
    freeTree(root->right);
    free(root);
}

这棵树的结构为

运行效果