Next Greater Element Java

PROGRAM TO FIND THE NEXT GREATER ELEMENT FOR EVERY ELEMENT



public class NGE {
    static class stack {
        int top;
        int items[] = new int[100];
 
        // Stack functions to be used by printNGE
        void push(int x)
        {
            if (top == 99)
            {
                System.out.println("Stack full");
            }
            else
            {
                items[++top] = x;
            }
        }
 
        int pop()
        {
            if (top == -1)
            {
                System.out.println("Underflow error");
                return -1;
            }
            else {
                int element = items[top];
                top--;
                return element;
            }
        }
 
        boolean isEmpty()
        {
            return (top == -1) ? true : false;
        }
    }
 
    /* prints element and NGE pair for
       all elements of arr[] of size n */
    static void printNGE(int arr[], int n)
    {
        int i = 0;
        stack s = new stack();
        s.top = -1;
        int element, next;
 
        /* push the first element to stack */
        s.push(arr[0]);
 
        // iterate for rest of the elements
        for (i = 1; i < n; i++)
        {
            next = arr[i];
 
            if (s.isEmpty() == false)
            {
 
                // if stack is not empty, then
                // pop an element from stack
                element = s.pop();
 
                /* If the popped element is smaller than
                   next, then a) print the pair b) keep
                   popping while elements are smaller and
                   stack is not empty */
                while (element < next)
                {
                    System.out.println(element + " --> "
                                       + next);
                    if (s.isEmpty() == true)
                        break;
                    element = s.pop();
                }
 
                /* If element is greater than next, then
                   push the element back */
                if (element > next)
                    s.push(element);
            }
 
            /* push next to stack so that we can find next
               greater for it */
            s.push(next);
        }
 
        /* After iterating over the loop, the remaining
           elements in stack do not have the next greater
           element, so print -1 for them */
        while (s.isEmpty() == false)
        {
            element = s.pop();
            next = -1;
            System.out.println(element + " -- " + next);
        }
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int arr[] = { 11, 13, 21, 3 };
        int n = arr.length;
        printNGE(arr, n);
    }
}


OUTPUT:
11 --> 13
13 --> 21
3 --> -1
21 --> -1

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