题解

#include<iostream>
#include<stack>
using namespace std;
int n,a[1005],vis[1005];
int main(){
    cin>>n;
	for(int i=1;i<=n;i++) cin>>a[i];
    stack<int> stk;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=a[i];j++){
            if(vis[j]==0){
                stk.push(j);
                vis[j]=1;
            }
        }
        if(stk.top() == a[i]){
            stk.pop();
        }
        else{
            cout<<"Impossible";
            return 0;
        }
    }
    cout<<"Possible";
    return 0;
} //rider 苏宇豪