SPOJ STPAR

// http://www.spoj.com/problems/STPAR/
#include <iostream>
#include <stack>

using namespace std;

int main() {
	ios_base::sync_with_stdio(false); 
	int num = 0;
	cin >> num;
	const int NOTSET = -1;

	stack<int> waitting;
	int car = 0;
	int lastcar = 0;
	bool carmoved = false;
	while (num != 0) {
		for (int i = 0; i < num; i++) {
			cin >> car;

			carmoved = false;
			while (true) {
				// first, try to move car from side street
				if (waitting.size()!=0 && (waitting.top() == (lastcar+1))) {
					lastcar = waitting.top();
					waitting.pop();
					continue;
				}
				// try move current car forward
				if ((lastcar+1) == car) {
					lastcar = car;
					carmoved = true;
					continue;
				}
				if (waitting.size()!=0 && (waitting.top() == (lastcar+1))) {
					continue;
				} else {
					if (!carmoved) waitting.push(car);
					break;
				}
			}
		}

		if (lastcar == num) {
			cout << "yes" << endl;
		} else {
			cout << "no" << endl;
		}


		waitting.empty();
		lastcar = 0;
		car = 0;
		cin >> num;
	}
}