用c语言代码编写二叉树的先序，中序，后序

二叉树的先序、中序、后序遍历可以使用递归的方式来实现，以下是使用 C 语言代码实现的示例：

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

// 定义二叉树结构体
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
};

// 先序遍历二叉树
void preOrder(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    printf("%d ", root->val);  // 先访问根节点
    preOrder(root->left);     // 再遍历左子树
    preOrder(root->right);    // 最后遍历右子树
}

// 中序遍历二叉树
void inOrder(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    inOrder(root->left);      // 先遍历左子树
    printf("%d ", root->val); // 再访问根节点
    inOrder(root->right);     // 最后遍历右子树
}

// 后序遍历二叉树
void postOrder(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    postOrder(root->left);    // 先遍历左子树
    postOrder(root->right);   // 再遍历右子树
    printf("%d ", root->val); // 最后访问根节点
}

// 创建二叉树
struct TreeNode* createTree() {
    struct TreeNode* root = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->val = 1;
    root->left = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->left->val = 2;
    root->left->left = NULL;
    root->left->right = NULL;
    root->right = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->right->val = 3;
    root->right->left = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->right->left->val = 4;
    root->right->left->left = NULL;
    root->right->left->right = NULL;
    root->right->right = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->right->right->val = 5;
    root->right->right->left = NULL;
    root->right->right->right = NULL;
    return root;
}

int main() {
    struct TreeNode* root = createTree();
    printf("Preorder traversal: ");
    preOrder(root);
    printf("\nInorder traversal: ");
    inOrder(root);
    printf("\nPostorder traversal: ");
    postOrder(root);
    printf("\n");
    return 0;
}

以上代码创建了一个二叉树并进行了先序、中序、后序遍历，输出结果为：

Preorder traversal: 1 2 3 4 5
Inorder traversal: 2 1 4 3 5
Postorder traversal: 2 4 5 3 1