LKS - Large Knapsack

LKS - Large Knapsack (link to the question)

can be implemented by using 0/1 knapsack with little bit modification since in each iteration two consecutive 

row are being used one is in read mode and another in write mode. Since the array could exceed the limit that why i did this modification... 

#include<stdio.h>
#define max(x,y) (x>y)?x:y;

int arr[2][2000001]={0};

struct node
{
    int value;
    int weight;
}obj[501];


int main()
{
    int i,j,n,W,k;

    scanf("%i%i",&W,&n);

    i=0;
    while(++i <= n)
        scanf("%i%i",&obj[i].value,&obj[i].weight);

    i=0;
    while(++i <= n)
    {
        j = 0;

        if(i%2)
        {
            while(++j <= W)
            {
                if(obj[i].weight <= j)
                    arr[1][j] = max( obj[i].value + arr[0][ j-obj[i].weight ] , arr[0][j] )
                else
                    arr[1][j] = arr[0][j];
            }

        }

        else
        {
            while(++j <= W)
            {
                if(obj[i].weight <= j)
                    arr[0][j] = max( obj[i].value + arr[1][ j-obj[i].weight ] , arr[1][j] )
                else
                    arr[0][j] = arr[1][j];
            }
        }
    }


    if(n&1)
        printf("%i\n",arr[1][W]);
    else
        printf("%i\n",arr[0][W]);

    return 0;
}