스택 활용

#include <stdio.h>
#include <iostream>
#include <algorithm>
#include <stack>
#include <vector>
#include <math.h>

using namespace std;


int main() {
	//freopen("input.txt.txt","rt",stdin);
	int n,i,arr[31],order=1,pos=1,cnt=0;
	scanf("%d", &n);
	for(i=1;i<=n;i++){
		scanf("%d", &arr[i]);
	}
	stack<int> s;
	string ans = "";
	for(i=1;i<=n;i++){
		s.push(arr[i]);
		ans+='P';
		
		while(!s.empty()&&s.top()==order){
			ans+='O';
			order++;
			s.pop();
		}
	}
	
	if(s.empty()) printf("%s\n",ans.c_str());
	else printf("impossible\n");

	return 0;	
}

기차 교차로 문제