# Count Numbers in Range with difference between Sum of digits at even and odd positions as Prime

Given a range [L, R]. The task is to count the numbers in the range having difference between the sum of digits at even position and sum of digits at odd position is a Prime Number. Consider the position of least significant digit in the number as an odd position.

Examples:

```Input : L = 1, R = 50
Output : 6
Explanation : Only, 20, 30, 31, 41,
42 and 50 are valid numbers.

Input : L = 50, R = 100
Output : 18
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Prerequisites :Digit DP

Approach : Firstly, if we are able to count the required numbers up to R i.e. in the range [0, R], we can easily reach our answer in the range [L, R] by solving for from zero to R and then subtracting the answer we get after solving from zero to L – 1. Now, we need to define the DP states.

DP States:

• Since we can consider our number as a sequence of digits, one state is the position at which we are currently at. This position can have values from 0 to 18 if we are dealing with the numbers up to 1018. In each recursive call, we try to build the sequence from left to right by placing a digit from 0 to 9.
• First state is the sum of the digits at even positions we have placed so far.
• Second state is the sum of the digits at odd positions we have placed so far.
• Another state is the boolean variable tight which tells the number we are trying to build has already become smaller than R so that in the upcoming recursive calls we can place any digit from 0 to 9. If the number has not become smaller, the maximum limit of digit we can place is digit at the current position in R.

Also, when we reach the base condition, we need to check whether the required difference is a prime number or not. Since the highest number in range is 1018, the maximum sum at either even or odd positions can be at max 9 times 9 and hence the maximum difference. So, we need to check only prime numbers only upto 100 at base condition.

Below is the implementation of the above approach:

 `// C++ implementation of the above approach ` ` `  `#include ` `using` `namespace` `std; ` ` `  `const` `int` `M = 18; ` `int` `a, b, dp[M]; ` ` `  `// Prime numbers upto 100 ` `int` `prime[] = { 2, 3, 5, 7, 11, 13, 17, 19, 23, ` `                ``29, 31, 37, 43, 47, 53, 59, 61,  ` `                ``67, 71, 73, 79, 83, 89, 97 }; ` ` `  `// Function to return the count of ` `// required numbers from 0 to num ` `int` `count(``int` `pos, ``int` `even, ``int` `odd, ``int` `tight, ` `        ``vector<``int``> num) ` `{ ` `    ``// Base Case ` `    ``if` `(pos == num.size()) { ` `        ``if` `(num.size() & 1) ` `            ``swap(odd, even); ` `        ``int` `d = even - odd; ` ` `  `        ``// check if the difference is equal ` `        ``// to any prime number ` `        ``for` `(``int` `i = 0; i < 24; i++) ` `            ``if` `(d == prime[i]) ` `                ``return` `1; ` `                 `  `        ``return` `0; ` `    ``} ` ` `  `    ``// If this result is already computed ` `    ``// simply return it ` `    ``if` `(dp[pos][even][odd][tight] != -1) ` `        ``return` `dp[pos][even][odd][tight]; ` ` `  `    ``int` `ans = 0; ` ` `  `    ``// Maximum limit upto which we can place ` `    ``// digit. If tight is 1, means number has ` `    ``// already become smaller so we can place ` `    ``// any digit, otherwise num[pos] ` `    ``int` `limit = (tight ? 9 : num[pos]); ` ` `  `    ``for` `(``int` `d = 0; d <= limit; d++) { ` `        ``int` `currF = tight, currEven = even; ` `        ``int` `currOdd = odd; ` `         `  `        ``if` `(d < num[pos]) ` `            ``currF = 1; ` ` `  `        ``// If the current position is odd ` `        ``// add it to currOdd, otherwise to ` `        ``// currEven ` `        ``if` `(pos & 1) ` `            ``currOdd += d; ` `        ``else` `            ``currEven += d; ` `             `  `        ``ans += count(pos + 1, currEven, currOdd, ` `                    ``currF, num); ` `    ``} ` `     `  `    ``return` `dp[pos][even][odd][tight] = ans; ` `} ` ` `  `// Function to convert x into its digit vector ` `// and uses count() function to return the ` `// required count ` `int` `solve(``int` `x) ` `{ ` `    ``vector<``int``> num; ` `     `  `    ``while` `(x) { ` `        ``num.push_back(x % 10); ` `        ``x /= 10; ` `    ``} ` `     `  `    ``reverse(num.begin(), num.end()); ` ` `  `    ``// Initialize dp ` `    ``memset``(dp, -1, ``sizeof``(dp)); ` `    ``return` `count(0, 0, 0, 0, num); ` `} ` ` `  `// Driver Code ` `int` `main() ` `{ ` `    ``int` `L = 1, R = 50; ` `    ``cout << solve(R) - solve(L - 1) << endl; ` ` `  `    ``L = 50, R = 100; ` `    ``cout << solve(R) - solve(L - 1) << endl; ` `     `  `    ``return` `0; ` `} `

 `// Java implementation of the above approach  ` `import` `java.util.*; ` ` `  `class` `GFG ` `{ ` ` `  `static` `int` `M = ``18``;  ` `static` `int` `a, b, dp[][][][] = ``new` `int``[M][``90``][``90``][``2``];  ` ` `  `// Prime numbers upto 100  ` `static` `int` `prime[] = { ``2``, ``3``, ``5``, ``7``, ``11``, ``13``, ``17``, ``19``, ``23``,  ` `                ``29``, ``31``, ``37``, ``43``, ``47``, ``53``, ``59``, ``61``,  ` `                ``67``, ``71``, ``73``, ``79``, ``83``, ``89``, ``97` `};  ` ` `  `// Function to return the count of  ` `// required numbers from 0 to num  ` `static` `int` `count(``int` `pos, ``int` `even, ``int` `odd, ``int` `tight,  ` `        ``Vector num)  ` `{  ` `    ``// Base Case  ` `    ``if` `(pos == num.size())  ` `    ``{  ` `        ``if` `((num.size() & ``1``) != ``0``) ` `        ``{  ` `            ``int` `t = odd; ` `            ``odd = even; ` `            ``even = t; ` `             `  `        ``} ` `        ``int` `d = even - odd;  ` ` `  `        ``// check if the difference is equal  ` `        ``// to any prime number  ` `        ``for` `(``int` `i = ``0``; i < ``24``; i++)  ` `            ``if` `(d == prime[i])  ` `                ``return` `1``;  ` `                 `  `        ``return` `0``;  ` `    ``}  ` ` `  `    ``// If this result is already computed  ` `    ``// simply return it  ` `    ``if` `(dp[pos][even][odd][tight] != -``1``)  ` `        ``return` `dp[pos][even][odd][tight];  ` ` `  `    ``int` `ans = ``0``;  ` ` `  `    ``// Maximum limit upto which we can place  ` `    ``// digit. If tight is 1, means number has  ` `    ``// already become smaller so we can place  ` `    ``// any digit, otherwise num[pos]  ` `    ``int` `limit = (tight != ``0` `? ``9` `: num.get(pos));  ` ` `  `    ``for` `(``int` `d = ``0``; d <= limit; d++)  ` `    ``{  ` `        ``int` `currF = tight, currEven = even;  ` `        ``int` `currOdd = odd;  ` `         `  `        ``if` `(d < num.get(pos))  ` `            ``currF = ``1``;  ` ` `  `        ``// If the current position is odd  ` `        ``// add it to currOdd, otherwise to  ` `        ``// currEven  ` `        ``if` `((pos & ``1``) != ``0``)  ` `            ``currOdd += d;  ` `        ``else` `            ``currEven += d;  ` `             `  `        ``ans += count(pos + ``1``, currEven, currOdd,  ` `                    ``currF, num);  ` `    ``}  ` `     `  `    ``return` `dp[pos][even][odd][tight] = ans;  ` `}  ` ` `  `// Function to convert x into its digit vector  ` `// and uses count() function to return the  ` `// required count  ` `static` `int` `solve(``int` `x)  ` `{  ` `    ``Vector num = ``new` `Vector();  ` `     `  `    ``while` `(x != ``0``)  ` `    ``{  ` `        ``num.add(x % ``10``);  ` `        ``x /= ``10``;  ` `    ``}  ` `     `  `    ``Collections.reverse(num);  ` ` `  `    ``// Initialize dp  ` `    ``for``(``int` `i = ``0``; i < dp.length; i++) ` `        ``for``(``int` `j = ``0``; j < dp[i].length; j++) ` `            ``for``(``int` `k = ``0``; k < dp[i][j].length; k++) ` `                ``for``(``int` `k1 = ``0``; k1 < dp[i][j][k].length; k1++) ` `                    ``dp[i][j][k][k1] = -``1``; ` `     `  `    ``return` `count(``0``, ``0``, ``0``, ``0``, num);  ` `}  ` ` `  `// Driver Code  ` `public` `static` `void` `main(String args[])  ` `{  ` `    ``int` `L = ``1``, R = ``50``;  ` `    ``System.out.println( solve(R) - solve(L - ``1``));  ` ` `  `    ``L = ``50``; R = ``100``;  ` `    ``System.out.println( solve(R) - solve(L - ``1``));  ` `}  ` `} ` ` `  `// This code is contributed by Arnab Kundu `

// C# implementation of the above approach
using System;
using System.Collections.Generic;

class GFG
{
static int M = 18;
static int a, b;
static int [,,,]dp = new int[M, 90, 90, 2];

// Prime numbers upto 100
static int []prime = { 2, 3, 5, 7, 11, 13, 17, 19, 23,
29, 31, 37, 43, 47, 53, 59, 61,
67, 71, 73, 79, 83, 89, 97 };

// Function to return the count of
// required numbers from 0 to num
static int count(int pos, int even,
int odd, int tight,
List num)
{
// Base Case
if (pos == num.Count)
{
if ((num.Count & 1) != 0)
{
int t = odd;
odd = even;
even = t;

}

int d = even – odd;

// check if the difference is equal
// to any prime number
for (int i = 0; i num = new List();

while (x != 0)
{
num.Add(x % 10);
x /= 10;
}

num.Reverse();

// Initialize dp
for(int i = 0; i Output:

```6
18
```

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