c语言创建一棵二叉树，对其实现先序、中序、后序遍历。

以下是C语言实现二叉树及遍历的示例代码：

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

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

// 创建一个二叉树节点
TreeNode* create_node(int val) {
    TreeNode* node = (TreeNode*)malloc(sizeof(TreeNode));
    node->val = val;
    node->left = NULL;
    node->right = NULL;
    return node;
}

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

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

// 中序遍历
void inorder_traversal(TreeNode* root) {
    if (root == NULL) {
        return;
    }
    inorder_traversal(root->left);
    printf("%d ", root->val);
    inorder_traversal(root->right);
}

// 后序遍历
void postorder_traversal(TreeNode* root) {
    if (root == NULL) {
        return;
    }
    postorder_traversal(root->left);
    postorder_traversal(root->right);
    printf("%d ", root->val);
}

int main() {
    TreeNode* root = build_tree();
    printf("Preorder Traversal: ");
    preorder_traversal(root);
    printf("\nInorder Traversal: ");
    inorder_traversal(root);
    printf("\nPostorder Traversal: ");
    postorder_traversal(root);
    return 0;
}

输出结果为：

Preorder Traversal: 1 2 4 5 3 6 7
Inorder Traversal: 4 2 5 1 6 3 7
Postorder Traversal: 4 5 2 6 7 3 1