Sam and sub-strings | Editorial by aksingh007

sumofdigit[i]  =  (i+1)*num[i] + 10*sumofdigit[i-1]
[see here in details](https://www.geeksforgeeks.org/sum-of-all-substrings-of-a-string-representing-a-number/)

```
import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;

public class Solution {
    static long mod=(long)(1000000000+7);
    
    static int substrings(String n,long arr[]) {
        // n="0"+n;
         long res=(long)(n.charAt(0)-48);
         int len=n.length();
        long sum[]=new long[len+1];
        sum[0]=res;
        // for(int i=1;i<n.length();i++){
        //     sum[i]=(sum[i-1]+n.charAt(i)-48)%mod;
        // }
        System.out.println(Arrays.toString(sum));
        for(int i=1;i<n.length();i++){
            int a=(int)n.charAt(i)-48;
            sum[i]=((i+1)*a)%mod+(10*sum[i-1])%mod;
            res=(res+sum[i])%mod;
            res%=mod;
        }
        return (int)( res%mod);
    }

private static final Scanner scanner = new Scanner(System.in);

public static void main(String[] args) throws IOException {
        BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));
        long arr[]=new long[20000005];
        long k=1L;
        for(int i=0;i<arr.length;i++){
            arr[i]=(k%mod);
            k=(k*10)%mod;
        }
        for(int i=1;i<arr.length;i++)
            arr[i]=(arr[i] +arr[i-1] )%mod;
        String n = scanner.nextLine();

int result = substrings(n,arr);

bufferedWriter.write(String.valueOf(result));
        bufferedWriter.newLine();

bufferedWriter.close();

scanner.close();
    }
}

```

Sam and sub-strings | Editorial by aksingh007

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