# 1.3 - Stack and Queue

### 1. 使用2個Stack製作Queue(Linklist)

概念示意圖：

![](/files/-LHCDgLgUyAGU4VJT3xz)

#### 1.1 Stack的pop()

```
int pop(stack **st){ //透過雙重指標改變指標變數的值
    stack *temp = *st;      //用來儲存stack的top節點
    if (IsEmpty(st) == 1){
        printf("The stack is empty.\n");
        return -100;
    }
    else{
        *st = temp->next;
    }
    return temp->Data;
}
```

#### 1.2 Stack的push()

```
void push(stack **st, int data){ //透過雙重指標改變指標變數的值
    stack *new_node = (stack *)malloc(sizeof(stack));
    
    // save the data to the new node
    new_node->Data = data;
    new_node->next = NULL;
    
    if (IsEmpty(st) == 1){
        *st = new_node;
    }
    else{
        new_node->next = *st; // the origin head is the older node
        *st = new_node; // the lastest data is the top node
    }
}
```

#### 1.3 Queue的enqueue()

```
void enqueue(int data){
    push(&s1, data); //改變s1的指標
}
```

#### 1.4 Queue的enqueue()

```
int dequeue(){
    int output;
    if (IsEmpty(&s1) == 1){
        printf("The stack is empty.\n");
        return 0;
    }
    else{ //執行概念示意圖的運作
        while(IsEmpty(&s1)!=1) 
            push(&s2, pop(&s1));
        output = pop(&s2);
        while(IsEmpty(&s2)!=1) 
            push(&s1, pop(&s2));
        return output; 
    }
    return -1;
}
```

#### 1.5 執行結果:

```
Enqueue: 1,	2, 3
//front(1, 2, 3)rear
Dequeue: 1
Dequeue: 2
Enqueue: 4
//front(3, 4)rear
Dequeue: 3
```

完整程式碼: <https://github.com/gary30404/Data-Structure/blob/master/1/1.4/queue_by_stack_in_linklist.c>

### 2. 使用1個Queue製作Stack(Linklist)

概念示意圖：

![](/files/-LHGLHBoG_mRN0orIq1P)

#### 2.1 Queue的dequeue()

和用Linklist做的Queue相同。

```
int dequeue(){
    if (isEmpty() == 1){
        printf("The queue is empty.\n");
        return 0;
    }
    else{
        queue *output = front;
        front = front->next; // the older data
        return output->Data; // return the top node
    }
    return -1;
}
```

#### 2.2 Queue的enqueue()

和用Linklist做的Queue相同。

```
void enqueue(int data){
    // request space for new node
    queue *new_node = (queue *)malloc(sizeof(queue));
    
    // save the data to the new node
    new_node->Data = data;
    new_node->next = NULL;
    
    // push to the back of the queue
    if (isEmpty() == 1){
        front = new_node;
        rear = front;
    }
    else{
        rear->next = new_node; // next of the rear node points to new node
        rear = new_node;
    }
}
```

#### 2.3 Stack的pop()

```
int pop(queue *q){
    queue *temp = q;
    if (q == NULL)
        printf("The stack is empty.\n");
    else{
        int ln = 0;
        while(q != NULL){ //計算Queue的大小
            ln++;
            q = q->next;
        }
        for (int i = 0; i<ln-1; i++){ //將n-1個資料dequeue()後再enqueue()，讓最後輸入的資料在Queue的front端
            int t = dequeue();
            enqueue(t);
        }
    }
    return dequeue();
}
```

#### 2.4 Stack的push()

```
void push(int data){
    enqueue(data); //將資料存到Queue中
}
```

#### 2.5 執行結果:

```
Push: 1, 2, 3
//(1, 2, 3)top
Pop: 3
Pop: 2
//(1)top
Push: 4, 5
//(1, 4, 5)top
Pop: 5
```

完整程式碼: <https://github.com/gary30404/Data-Structure/blob/master/1/1.4/stack_by_queue_in_linklist.c>


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