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Pick from both sides?

Stack Overflow Asked by helloworld on December 13, 2020

The problem statement is :
Given an integer array A of size N.

You can pick B elements from either left or right end of the array A to get maximum sum.
Find and return this maximum possible sum.

NOTE: Suppose B = 4 and array A contains 10 elements then:

You can pick first four elements or can pick last four elements or can pick 1 from front and 3 from back etc . you need to return the maximum possible sum of elements you can pick.

public class Solution {
ArrayList<Integer> c = new ArrayList<>();
ArrayList<Integer> A= new ArrayList<>();
public int solve(ArrayList<Integer> A, int B) {
   
    if (B>A.size()){
        int sum=0;
        for(int i=0;i<A.size();i++)
         sum= sum+A.get(i);
        return sum;
    }

   int max_sum=0;
 for(int i=0;i<A.size();i++){
  if((max_sum<suffix(A.size()-(B-i))+prefix(i-1)) ){
       max_sum=suffix(A.size()-(B-i))+prefix(i-1);
     }
    }
    return max_sum;
}
    
    int prefix_sum=0;
  int prefix(int a)   {
      
       for(int p=0;p<a+1;p++){
           c=A;
            prefix_sum=prefix_sum + c.get(p);
            }
          return prefix_sum;
                     }

           int suffix_sum=0;
           int suffix(int b){
               c=A;
        for(int q=b;q<c.size();q++){
           suffix_sum=suffix_sum+c.get(q);
          }
            return suffix_sum;
         }

}

I am getting runtime error, I have tried to implement the suffix and prefix methods which return the sum from the index[ 0, i] and sum from [i, N-i] respectively, then in the solve function I am trying to find the sum of prefix [a-1] +suffix[N-(b-a)] and find out the maximum sum, the syntax is completely correct, there is something wrong with the logic I assume, please help me find the correct solution by correcting this code instead of providing an alternative method

2 Answers

    package com.array;

    import java.util.Arrays;
    import java.util.List;

    public class PickFromBothSides {

        public static void main(String[] args) {
            Integer[] arr = { 5, -2, 3, 1, 2 };
            System.out.println(solve(Arrays.asList(arr), 3));

        }

        public static int solve(List<Integer> A, int B) {

            int n = A.size();

            int result = 0;

            for (int i = 0; i < B; i++) {
                result += A.get(i);
            }

            int sum = result;

            for (int i = 0; i < B; i++) {
                sum -= A.get(B - 1 - i);
                sum += A.get(n - 1 - i);

                result = Math.max(result, sum);
            }

            return result;

        }
    }

Runtime O(n) Space complexity O(1)

Answered by vaquar khan on December 13, 2020

You are declaring int prefix_sum=0; and int suffix_sum=0; as fields, not as local variables of the respective methods.

You are calling suffix(A.size()-(B-i)) so with your example that is 10 - (4 -i) which is 6 + i. You iterate through i being in the range {0, ..., 10} so the value 6 + i will be all the numbers 6 through 16. You cannot index in the array above 9, so you get an exception.

You need to change

for(int i=0;i<A.size();i++){

to

for(int i=0; i <= B; i++){

because you are trying to ask each iteration "how many numbers are taken from the beginning"? 0, 1, 2, 3 or 4 if B is 4


Other upgrades:

  1. You are calling suffix(A.size()-(B-i))+prefix(i-1)) twice in a row. Call it only once, store it in a variable and reuse.

  2. You are calling prefix(i-1) but inside prefix() you are using the parameter a as a + 1. You don't need to subtract one and add one to the same thing

Answered by Hawk on December 13, 2020

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