Sublists of a list in java

Question

There are N buildings in a certain one-dimensional landscape. Each building has a height given by hi,i∈[1,N]. If you join K adjacent buildings, they will form a solid rectangle of area K×min(hi,hi+1,…,hi+k−1).

Given N buildings, find the greatest such solid area formed by consecutive buildings.

Input Format The first line contains N, the number of buildings altogether. The second line contains N space-separated integers, each representing the height of a building.

Output Format One integer representing the maximum area of rectangle formed.

Sample Input

5
1 2 3 4 5

Sample Output

9

This is my code , i tried to cross check it but there was a problem with it , whenever i access a sublist , it is getting sorted automatically and their positions are aso getting changed in the original list

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

 public class Solution 
{
 public static void main(String[] args) 
 {
   Scanner sc = new Scanner(System.in);
   int n=sc.nextInt();

   List<Integer> al= new ArrayList<Integer>();

    for(int i=0;i<n;i++)
        al.add(sc.nextInt());

  int i, c,min=0;

  for( c = 0 ; c < n ; c++ )
  {

     for( i = c+1 ; i <= n ; i++ )
     {   
         System.out.println(" value of c is "+c+" value of i is "+i);
         List<Integer> X = al.subList(c,i);

         for(int j=0;j<X.size();j++)
             System.out.print(X.get(j)+" ");

         System.out.println();

         Collections.sort(X);

         for(int j=0;j<X.size();j++)
             System.out.print(X.get(j)+" ");

         System.out.println();

         int x=X.get(0);
         System.out.println("min value is "+x);
         int y=x*(X.size());
         System.out.println("projected value is "+y);

         if(y > min)
             min = y;

         System.out.println("modified value is "+min);

      }
   }

    System.out.println(min);
  }
}

Answer 1

The subList() method is actually explicitly documented to give you a view of the original:

Returns a view of the portion of this list between the specified * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. (If * {@code fromIndex} and {@code toIndex} are equal, the returned list is * empty.) The returned list is backed by this list, so non-structural * changes in the returned list are reflected in this list, and vice-versa. * The returned list supports all of the optional list operations.

So subList() is not suitable if you want to modify it and not reflect the changes back to the original.

Instead make an explicit copy:

List<Integer> X = new ArrayList<>(al.subList(c,i));