二叉树先序遍历用goto语句实现

二叉树的先序遍历可以使用递归或非递归的方式实现，使用`goto`语句也可以完成。以下是使用`goto`语句实现二叉树先序遍历的示例代码：

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

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

void preorderTraversal(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    
    struct TreeNode* stack[1000];  // 定义一个栈用于存储待访问的节点
    int top = -1;  // 栈顶指针初始化为-1
    struct TreeNode* node = root;
    
    while (1) {
        while (node != NULL) {
            printf("%d ", node->val);  // 访问当前节点的值
            stack[++top] = node;  // 将当前节点入栈
            node = node->left;  // 继续遍历左子树
        }
        
        if (top != -1) {
            node = stack[top--];  // 弹出栈顶节点
            node = node->right;  // 遍历右子树
        } else {
            break;
        }
    }
}

int main() {
    // 构建一个二叉树
    struct TreeNode* root = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    struct TreeNode* node1 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    struct TreeNode* node2 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    struct TreeNode* node3 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    struct TreeNode* node4 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    
    root->val = 1;
    node1->val = 2;
    node2->val = 3;
    node3->val = 4;
    node4->val = 5;
    
    root->left = node1;
    root->right = node2;
    node1->left = node3;
    node1->right = NULL;
    node2->left = node4;
    node2->right = NULL;
    node3->left = NULL;
    node3->right = NULL;
    node4->left = NULL;
    node4->right = NULL;
    
    // 先序遍历二叉树
    printf("Preorder traversal:\n");
    preorderTraversal(root);
    
    // 释放内存
    free(node4);
    free(node3);
    free(node2);
    free(node1);
    free(root);
    
    return 0;
}