https://www.acmicpc.net/problem/18258

큐 2.c

#include <stdio.h>
#include <string.h>

int queue[2000001];
int front=0;
int rear=-1;

void clean(char arr[]); // 초기화용 함수
void push(int x);
void pop(void);
void size(void);
void empty(void);
int main(void){
	int n, i, x;
	char command[6];
	scanf("%d", &n);
	for(i=0;i<n;i++){
		scanf("%s", command); // 명령 입력
		if(!strcmp(command,"push")){
			scanf("%d", &x);
			push(x);
		}else if(!strcmp(command,"pop"))
			pop();
		else if(!strcmp(command,"size"))
			size();
		else if(!strcmp(command,"empty"))
			empty();
		else if(!strcmp(command,"front")){
			if(rear-front+1==0)
				printf("%d\n", -1);
			else
				printf("%d\n", queue[front]);
		}
		else if(!strcmp(command,"back")){
			if(rear-front+1==0)
				printf("%d\n", -1);
			else
				printf("%d\n", queue[rear]);
		}
		clean(command); // 초기화
	}
	return 0;
}
void clean(char arr[]){
	int i;
	for(i=0;i<6;i++)
		arr[i]='\0';
}
void push(int x){
	queue[++rear]=x;
}
void pop(void){
	if(rear-front+1==0)
		printf("%d\n", -1);
	else
		printf("%d\n", queue[front++]);
}
void size(void){
	printf("%d\n", rear-front+1);
}
void empty(void){
	if(rear-front+1!=0)
		printf("%d\n", 0);
	else
		printf("%d\n", 1);
}