用C语言编写一算法，判别给定二叉树是否为完全二叉树。

以下是使用C语言编写的判断给定二叉树是否为完全二叉树的算法：

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

struct Node {
    int data;
    struct Node* left;
    struct Node* right;
};

bool isCompleteBinaryTree(struct Node* root) {
    if (root == NULL) {
        return true;
    }
    bool end = false;
    struct Node* queue[1000]; // 假设二叉树节点数不超过1000
    int front = 0, rear = 0;
    queue[rear++] = root;
    while (front < rear) {
        struct Node* node = queue[front++];
        if (node->left != NULL) {
            if (end) {
                return false;
            }
            queue[rear++] = node->left;
        } else {
            end = true;
        }
        if (node->right != NULL) {
            if (end) {
                return false;
            }
            queue[rear++] = node->right;
        } else {
            end = true;
        }
    }
    return true;
}

int main() {
    // 构造一棵完全二叉树
    struct Node* root = (struct Node*) malloc(sizeof(struct Node));
    root->data = 1;
    root->left = (struct Node*) malloc(sizeof(struct Node));
    root->left->data = 2;
    root->left->left = (struct Node*) malloc(sizeof(struct Node));
    root->left->left->data = 4;
    root->left->left->left = NULL;
    root->left->left->right = NULL;
    root->left->right = (struct Node*) malloc(sizeof(struct Node));
    root->left->right->data = 5;
    root->left->right->left = NULL;
    root->left->right->right = NULL;
    root->right = (struct Node*) malloc(sizeof(struct Node));
    root->right->data = 3;
    root->right->left = (struct Node*) malloc(sizeof(struct Node));
    root->right->left->data = 6;
    root->right->left->left = NULL;
    root->right->left->right = NULL;
    root->right->right = NULL;

    // 判断是否为完全二叉树
    bool isComplete = isCompleteBinaryTree(root);
    if (isComplete) {
        printf("The binary tree is a complete binary tree.\n");
    } else {
        printf("The binary tree is not a complete binary tree.\n");
    }

    return 0;
}

该算法首先检查树是否为空。如果为空，则它是一棵完全二叉树。如果不为空，则使用一个队列来遍历该树。如果一个节点有左子节点但没有右子节点，或者一个节点没有左子节点和右子节点，则接下来遍历的节点必须都是叶子节点。如果一个节点已经满足这个条件，但接下来遍历的节点不都是叶子节点，则该树不是完全二叉树。否则，该树是一棵完全二叉树。