Test.java

/* https://www.spoj.com/problems/STPAR/
 * #queue #stack #ad-hoc-1 #todo
 */
import java.util.Deque;
import java.util.LinkedList;
import java.util.Scanner;

class Test {
  static String calSub(int[] arr) {
    Deque<Integer> sideStreet = new LinkedList<Integer>(); // stack
    int lastCar = 0;

    for (int i = 0; i < arr.length; i++) {
      if (arr[i] == 1) {
        // mainStreet.add(arr[i]);
        lastCar = 1;
      } else if (lastCar != 0) {
        while (!sideStreet.isEmpty() && sideStreet.peekLast() == lastCar + 1) {
          sideStreet.pollLast();
          lastCar++;
        }
        if (!sideStreet.isEmpty() && arr[i] > sideStreet.peekLast()) {
          return "no";
        }
        if (arr[i] == lastCar + 1) {
          lastCar++;
        } else {
          sideStreet.addLast(arr[i]);
        }
      } else {
        if (!sideStreet.isEmpty() && sideStreet.peekLast() < arr[i]) {
          return "no";
        }
        sideStreet.addLast(arr[i]);
      }
    }
    if (!sideStreet.isEmpty() && sideStreet.peekLast() < lastCar) {
      return "no";
    }

    return "yes";
  }

  static void calculate() {
    Scanner sc = new Scanner(System.in);
    while (true) {
      int n = sc.nextInt();
      if (n == 0) {
        break;
      }
      int[] arr = new int[n];
      for (int i = 0; i < n; i++) {
        int temp = sc.nextInt();
        arr[i] = temp;
      }
      System.out.println(calSub(arr));
    }
  }

  public static void main(String[] args) {
    calculate();
  }
}