c语言使用队列按层次遍历建立二叉树

在C语言中，我们可以使用队列按层次遍历建立二叉树。

具体实现步骤如下：

1. 定义二叉树节点结构体：

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

2. 定义队列结构体：

typedef struct QueueNode {
    TreeNode *treenode;
    struct QueueNode *next;
} QueueNode;

typedef struct Queue {
    QueueNode *front;
    QueueNode *rear;
} Queue;

3. 实现队列的入队、出队、判空操作：

void enqueue(Queue *q, TreeNode *treenode) {
    QueueNode *node = (QueueNode *)malloc(sizeof(QueueNode));
    node->treenode = treenode;
    node->next = NULL;

    if (q->front == NULL) {
        q->front = node;
    } else {
        q->rear->next = node;
    }

    q->rear = node;
}

TreeNode *dequeue(Queue *q) {
    if (q->front == NULL) {
        return NULL;
    }

    TreeNode *treenode = q->front->treenode;
    QueueNode *temp = q->front;
    q->front = q->front->next;
    free(temp);

    return treenode;
}

bool isEmpty(Queue *q) {
    return q->front == NULL;
}

4. 实现按层次遍历建立二叉树的函数：

TreeNode *createBinaryTreeByLevelOrder(void) {
    Queue *q = (Queue *)malloc(sizeof(Queue));
    q->front = q->rear = NULL;

    TreeNode *root = NULL;

    int val;
    printf("请输入根节点的值：");
    scanf("%d", &val);

    if (val == -1) {
        return NULL;
    }

    root = (TreeNode *)malloc(sizeof(TreeNode));
    root->val = val;
    root->left = root->right = NULL;

    enqueue(q, root);

    while (!isEmpty(q)) {
        TreeNode *parent = dequeue(q);

        printf("请输入%d节点的左子节点的值：", parent->val);
        scanf("%d", &val);

        if (val != -1) {
            TreeNode *left = (TreeNode *)malloc(sizeof(TreeNode));
            left->val = val;
            left->left = left->right = NULL;
            parent->left = left;
            enqueue(q, left);
        }

        printf("请输入%d节点的右子节点的值：", parent->val);
        scanf("%d", &val);

        if (val != -1) {
            TreeNode *right = (TreeNode *)malloc(sizeof(TreeNode));
            right->val = val;
            right->left = right->right = NULL;
            parent->right = right;
            enqueue(q, right);
        }
    }

    return root;
}

完整代码如下：