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# Implement Queue with LL

### Explanation

* // Definition:
*
* back front
* v v
* Q: "1" <- "2" <- "3"
*
*
* // enQueue: Push an element from the "back"
*
* back front
* v v
* Q: "1" <- "2" <- "3"
*
* Step 1: create a Node: new = "0"
*
* Step 2: add from the back, therefore,
* new->next = back
*
* back front
* v v
* "0" -> "1" <- "2" <- "3"
*
* Step 3: back->next = new
*
* |---- back front
* v v v
* "0" "1" <- "2" <- "3"
*
* Step 4: back = new
*
* back front
* v v
* "0" <- "1" <- "2" <- "3"
*
*
* // deQueue: Delete an element from the "front"
*
* back front
* v v
* Q: "1" <- "2" <- "3"
*
* Step 1: create a Node: tmp to point "front"
*
* front
* back tmp
* v v
* Q: "1" <- "2" <- "3"
*
*
* Step 2: q->front = q->front->next
*
*
* back front tmp
* v v v
* "1" <- "2" <- "3"
*
* Step 3: free(tmp)
*
* back front
* v v
* "1" -> "2"
*

### C version

#### Declaration data structures:

#include <stdio.h>
#include <stdlib.h>
struct Node{
struct Node* next;
int val;
};
struct Queue{
struct Node* front;
struct Node* back;
};

#### Create Queue and New an element

struct Node* newNode(int value){
struct Node* new_node = (struct Node*) malloc (sizeof(struct Node));
new_node->val = value;
new_node->next = NULL;
return new_node;
}
struct Queue* creatQueue(){
struct Queue* q = (struct Queue*) malloc (sizeof(struct Queue));
q->front = NULL;
q->back = NULL;
return q;
}

#### Enqueue and Dequeue

void deQueue(struct Queue *q){
if(q->front == NULL){
return;
}
struct Node* temp = q->front;
q->front = q->front->next;
if (q->front == NULL)
q->back = NULL;
free(temp);
}
void enQueue(struct Queue *q, int value){
struct Node* new_node = newNode(value);
if(q->back == NULL){
q->front = new_node;
q->back = new_node;
return;
}
q->back->next = new_node;
q->back = new_node;
}

#### Test code:

int main() {
struct Queue* q = creatQueue();
enQueue(q, 10);
enQueue(q, 20);
deQueue(q);
deQueue(q);
enQueue(q, 30);
enQueue(q, 40);
enQueue(q, 50);
deQueue(q);
printf("Queue Front : %d \n", q->front->val);
printf("Queue back : %d", q->back->val);
return 0;
}