队列是包含元素集合的抽象数据结构。队列执行FIFO机制，即，首先插入的元素也将首先删除。换句话说，最近最少添加的元素首先在队列中被删除。
使用链表实现队列的程序如下：
#include <iostream>
using namespace std;
struct node {
int data;
struct node *next;
};
struct node* front = NULL;
struct node* rear = NULL;
struct node* temp;
void Insert() {
int val;
cout<<"Insert the element in queue : "<<endl;
cin>>val;
if (rear == NULL) {
rear = (struct node *)malloc(sizeof(struct node));
rear->next = NULL;
rear->data = val;
front = rear;
} else {
temp=(struct node *)malloc(sizeof(struct node));
rear->next = temp;
temp->data = val;
temp->next = NULL;
rear = temp;
}
}
void Delete() {
temp = front;
if (front == NULL) {
cout<<"Underflow"<<endl;
return;
}
else
if (temp->next != NULL) {
temp = temp->next;
cout<<"Element deleted from queue is : "<<front->data<<endl;
free(front);
front = temp;
} else {
cout<<"Element deleted from queue is : "<<front->data<<endl;
free(front);
front = NULL;
rear = NULL;
}
}
void Display() {
temp = front;
if ((front == NULL) && (rear == NULL)) {
cout<<"Queue is empty"<<endl;
return;
}
cout<<"Queue elements are: ";
while (temp != NULL) {
cout<<temp->data<<" ";
temp = temp->next;
}
cout<<endl;
}
int main() {
int ch;
cout<<"1) Insert element to queue"<<endl;
cout<<"2) Delete element from queue"<<endl;
cout<<"3) Display all the elements of queue"<<endl;
cout<<"4) Exit"<<endl;
do {
cout<<"Enter your choice : "<<endl;
cin>>ch;
switch (ch) {
case 1: Insert();
break;
case 2: Delete();
break;
case 3: Display();
break;
case 4: cout<<"Exit"<<endl;
break;
default: cout<<"Invalid choice"<<endl;
}
} while(ch!=4);
return 0;
}输出结果
上面程序的输出如下
1) Insert element to queue 2) Delete element from queue 3) Display all the elements of queue 4) Exit Enter your choice : 1 Insert the element in queue : 4 Enter your choice : 1 Insert the element in queue : 3 Enter your choice : 1 Insert the element in queue : 5 Enter your choice : 2 Element deleted from queue is : 4 Enter your choice : 3 Queue elements are : 3 5 Enter your choice : 7 Invalid choice Enter your choice : 4 Exit
在上面的程序中，该函数Insert()将元素插入队列。如果Rear为NULL，则队列为空，并插入单个元素。否则，在后方插入具有所需元素的节点，然后将该节点设置为后方。这如下所示-
void Insert() {
int val;
cout<<"Insert the element in queue : "<<endl;
cin>>val;
if (rear == NULL) {
rear = (struct node *)malloc(sizeof(struct node));
rear->next = NULL;
rear->data = val;
front = rear;
} else {
temp=(struct node *)malloc(sizeof(struct node));
rear->next = temp;
temp->data = val;
temp->next = NULL;
rear = temp;
}
}在函数中Delete()，如果队列中没有元素，则为下溢状态。如果队列中只有一个元素被删除，并且前台和后方设置为NULL。否则，将删除前面的元素，并且前面的元素指向下一个元素。这如下所示-
void Delete() {
temp = front;
if (front == NULL) {
cout<<"Underflow"<<endl;
return;
} else
if (temp->next != NULL) {
temp = temp->next;
cout<<"Element deleted from queue is : "<<front->data<<endl;
free(front);
front = temp;
} else {
cout<<"Element deleted from queue is : "<<front->data<<endl;
free(front);
front = NULL;
rear = NULL;
}
}在函数中display()，如果front和tail为NULL，则queue为空。否则，将在temp变量的帮助下使用while循环显示所有队列元素。这如下所示-
void Display() {
temp = front;
if ((front == NULL) && (rear == NULL)) {
cout<<"Queue is empty"<<endl;
return;
}
cout<<"Queue elements are: ";
while (temp != NULL) {
cout<<temp->data<<" ";
temp = temp->next;
}
cout<<endl;
}main()如果用户要插入，删除或显示队列，该功能将为用户提供一个选择。根据用户响应，使用开关调用适当的功能。如果用户输入无效的响应，则将其打印出来。下面的代码片段如下-
int main() {
int ch;
cout<<"1) Insert element to queue"<<endl;
cout<<"2) Delete element from queue"<<endl;
cout<<"3) Display all the elements of queue"<<endl;
cout<<"4) Exit"<<endl;
do {
cout<<"Enter your choice : "<<endl;
cin>>ch;
switch (ch) {
case 1: Insert();
break;
case 2: Delete();
break;
case 3: Display();
break;
case 4: cout<<"Exit"<<endl;
break;
default: cout<<"Invalid choice"<<endl;
}
} while(ch!=4);
return 0;
}