用c语言编写代码并标出注释：通过输入一棵二叉树的层次遍历序列构造一棵二叉树

以下是用 C 语言编写的代码，用于通过输入一棵二叉树的层次遍历序列构造一棵二叉树：

#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;

// 定义队列结构体
typedef struct Queue {
    TreeNode **data;
    int front;
    int rear;
    int size;
} Queue;

// 初始化队列
Queue *initQueue(int size) {
    Queue *queue = (Queue *) malloc(sizeof(Queue));
    queue->data = (TreeNode **) malloc(sizeof(TreeNode *) * size);
    queue->front = 0;
    queue->rear = 0;
    queue->size = size;
    return queue;
}

// 入队
void enqueue(Queue *queue, TreeNode *node) {
    if (queue->rear == queue->size) {
        printf("Queue is full.\n");
        return;
    }
    queue->data[queue->rear++] = node;
}

// 出队
TreeNode *dequeue(Queue *queue) {
    if (queue->front == queue->rear) {
        printf("Queue is empty.\n");
        return NULL;
    }
    return queue->data[queue->front++];
}

// 构造二叉树
TreeNode *buildTree(int *nums, int numsSize) {
    if (numsSize == 0) {
        return NULL;
    }
    // 创建根节点
    TreeNode *root = (TreeNode *) malloc(sizeof(TreeNode));
    root->val = nums[0];
    root->left = NULL;
    root->right = NULL;
    // 创建队列
    Queue *queue = initQueue(numsSize);
    enqueue(queue, root);
    // 遍历数组
    int i = 1;
    while (i < numsSize) {
        // 取出队首元素
        TreeNode *node = dequeue(queue);
        // 创建左子节点
        if (i < numsSize && nums[i] != -1) {
            TreeNode *left = (TreeNode *) malloc(sizeof(TreeNode));
            left->val = nums[i];
            left->left = NULL;
            left->right = NULL;
            node->left = left;
            enqueue(queue, left);
        }
        i++;
        // 创建右子节点
        if (i < numsSize && nums[i] != -1) {
            TreeNode *right = (TreeNode *) malloc(sizeof(TreeNode));
            right->val = nums[i];
            right->left = NULL;
            right->right = NULL;
            node->right = right;
            enqueue(queue, right);
        }
        i++;
    }
    return root;
}

// 打印二叉树
void printTree(TreeNode *root) {
    if (root == NULL) {
        return;
    }
    printf("%d ", root->val);
    printTree(root->left);
    printTree(root->right);
}

int main() {
    int nums[] = {1, 2, 3, 4, -1, 5, 6, -1, -1, 7, -1, -1, -1, -1, -1};
    int numsSize = sizeof(nums) / sizeof(int);
    TreeNode *root = buildTree(nums, numsSize);
    printTree(root);
    return 0;
}

注释已经标出，代码中使用了队列来辅助构造二叉树。