用c语言以二叉链表表示二叉树，建立一棵二叉树

以下是用C语言以二叉链表表示二叉树，建立一棵二叉树的示例代码：

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

typedef struct TreeNode {
    int val;
    struct TreeNode* left;
    struct TreeNode* right;
} TreeNode;

// 创建一个新的节点
TreeNode* createNode(int val) {
    TreeNode* newNode = (TreeNode*)malloc(sizeof(TreeNode));
    newNode->val = val;
    newNode->left = NULL;
    newNode->right = NULL;
    return newNode;
}

// 构建二叉树
TreeNode* buildTree() {
    TreeNode* root = createNode(1);
    root->left = createNode(2);
    root->right = createNode(3);
    root->left->left = createNode(4);
    root->left->right = createNode(5);
    root->right->left = createNode(6);
    root->right->right = createNode(7);
    return root;
}

// 先序遍历二叉树
void preOrderTraversal(TreeNode* root) {
    if (root == NULL) {
        return;
    }
    printf("%d ", root->val);
    preOrderTraversal(root->left);
    preOrderTraversal(root->right);
}

int main() {
    TreeNode* root = buildTree();
    printf("Preorder traversal of the binary tree is: ");
    preOrderTraversal(root);
    printf("\n");
    return 0;
}

输出结果为：`Preorder traversal of the binary tree is: 1 2 4 5 3 6 7`。