Coding Challenge Day- 31: Problem 1: Nearest Greatest Element to the Left in an Integer array

Hello! So I’m doing a 30 day coding challenge where I solve a few questions every day and thought of posting them here on my blog so that you guys can join the challenge too!

Welcome to Coding challenge Day 31: Problem 1! Be sure to post your answers, queries etc in the comments!

Problem : Nearest Greatest Element to the Left in an Integer array: Given an array, print the Next Greater Element for every element. The Next greater Element for an element x is the first greater element on the left side of x in array. Elements for which no greater element exist, consider next greater element as -1.

Sample: int [] array= {1,5,2,4};
Output: -1 -1 5 5

Sample: int [] array= {1,5,2,4,9,7,10,3,29,12};
Output: -1 -1 5 5 -1 9 -1 10 -1 29

Solution:

View/ Download from Github: (LINK)

package stacks;

import java.util.Stack;
import java.util.Vector;

public class Nearest_greater_to_left {
	
	public static Vector ngl(int [] array, int length){ 
		Vector v= new Vector();
		Stack s= new Stack();
		
		for (int i=0; i<length; i++){
			if (s.isEmpty()){
				v.add(-1);
			}
			
			else if((int)(s.peek())>array[i]){
				v.add(s.peek());
			}
			
			else if ((int)(s.peek())<array[i]){
				while(!s.isEmpty() && (int)(s.peek())<=array[i]){
					s.pop();
				}
				
				if(s.isEmpty()){
					v.add(-1);
				} else {
					v.add(s.peek());
				}
			}
			
			s.push(array[i]);
		}
		
		return v;
	}
	
	public static void main (String [] args){
		int [] array= {1,5,2,4,9,7,10,3,29,12};
		Vector v= ngl(array, array.length);
		
		for(Object obj: v){
			System.out.print(obj +" ");
		}
	}

}

Happy Learning!!