Question

http://en.wikipedia.org/wiki/All_nearest_smaller_values. This is the site of the problem
and here is my code, but I have some trouble to implement it:

import java.util.*;
public class stack{

    public static void main(String[]args){

        int x[]=new int[]{  0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15 };

        Stack<Integer> st=new Stack<Integer>();

        for (int a:x){
            while (!st.empty() && st.pop()>=a){
                System.out.println( st.pop());
                if (st.empty()){
                    break;
                }
                else{
                    st.push(a);
                }
            }
        }
    }
}

And here is the pseudo code from the site:

S = new empty stack data structure
for x in the input sequence:
    while S is nonempty and the top element of S is greater than or equal to x:
        pop S
    if S is empty:
        x has no preceding smaller value
    else:
        the nearest smaller value to x is the top element of S
    push x onto S

What is the matter with my code?

Answer 1

Here is the same pseudo code but I’ve added brackets so you can see where each statement begins and ends.

S = new empty stack data structure
for x in the input sequence:
{
    // peek instead of pop when you're checking what's in the queue
    while S is nonempty and the top element of S is greater than or equal to x:
    {
        pop S // you can call pop here
    }

    if S is empty:  // just check if the queue is empty, don't peek or pop
    {
        x has no preceding smaller value
    }
    else:
    {
        the nearest smaller value to x is the top element of S
    }
    push x onto S
}

You had the if/else statement inside the while loop, which was incorrect.

Check the documentation of the stack to understand what push, pop and peek do, here is the documentation: http://java.sun.com/j2se/1.4.2/docs/api/java/util/Stack.html