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JAVA EXAMPLE PROGRAMS

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Program: HackerRank stack problem - Find maximum element.


Problem Description:

Problem Reference: Maximum Element

You have an empty sequence, and you will be given N queries. Each query is one of these three types:

  • 1 x -Push the element x into the stack.
  • 2 -Delete the element present at the top of the stack.
  • 3 -Print the maximum element in the stack.

Input Format

The first line of input contains an integer, N. The next N lines each contain an above mentioned query. (It is guaranteed that each query is valid.)

Output Format

For each type 3 query, print the maximum element in the stack on a new line.

Sample Input

10
1 97
2
1 20
2
1 26
1 20
2
3
1 91
3

Sample Output

26
91

Our Approach

Our approach is pretty straight forward. We will use two stacks here. The first stack is to keeping track of all the elements to perform push and pop (element stack). And the other is for keeping track of the maximum element (maximum elements stack).

When you read the first element, push it onto both of the stacks.

We maintain the maximum elements stack so that the maximum element till now is on the top. Whenever we push an element, it normally goes to the elements stack, but we also push it to the maximum elements stack if, and only if, it is greater than the current maximum element.

Now, when an element is to be deleted, we pop it directly from the elements stack. We also check if that particular element is the maximum element or not. We do so by comparing the value and the index of the popped element with the element on top of the maximum elements stack. If they are equal, we pop that element as well.

Finally, to answer the maximum element query 3, we print the element on top of the maximum elements stack.



StackMaxElement
package com.java2novice.algos;

import java.util.Scanner;
import java.util.Stack;

public class StackMaxElement {

    public static void main(String arg[]) {

        Stack<Integer> stack = new Stack<>();
        Stack<Integer> maxStack = new Stack<>();

        Scanner snr = new Scanner(System.in);
        int n = snr.nextInt();
        for(int i=0;i<n;i++) {
            int input = snr.nextInt();
            switch (input){
                case 1:
                    int x = snr.nextInt();
                    stack.push(x);
                    if(maxStack.isEmpty() || x >= maxStack.peek()) {
                        maxStack.push(x);
                    }
                    break;

                case 2:
                    int popVl = stack.pop();
                    if(popVl == maxStack.peek()) {
                        maxStack.pop();
                    }
                    break;

                case 3:
                    System.out.println(maxStack.peek());
                    break;

                default: break;
            }
        }
        snr.close();
    }
}

Output:
---User Input---
10
1 97
2
1 20
2
1 26
1 20
2
3
1 91
3
---Program Output---
26

91
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Knowledge Centre
Java class can be private?
We can not declare top level class as private. Java allows only public and default modifier for top level classes in java. Inner classes can be private.
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About Author

I'm Nataraja Gootooru, programmer by profession and passionate about technologies. All examples given here are as simple as possible to help beginners. The source code is compiled and tested in my dev environment.

If you come across any mistakes or bugs, please email me to [email protected].

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