Given a number n, we need to count total number of n digit numbers such that the sum of even digits is 1 more than the sum of odd digits. Here even and odd means positions of digits are like array indexes, for exampl, the leftmost (or leading) digit is considered as even digit, next to leftmost is considered as odd and so on.

Example

Input: n = 2 Output: Required Count of 2 digit numbers is 9 Explanation : 10, 21, 32, 43, 54, 65, 76, 87, 98. Input: n = 3 Output: Required Count of 3 digit numbers is 54 Explanation: 100, 111, 122, ......, 980

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This problem is mainly an extension of Count of n digit numbers whose sum of digits equals to given sum. Here the solution of subproblems depend on four variables: digits, esum (current even sum), osum (current odd sum), isEven(A flag to indicate whether current digit is even or odd).

Below is Memoization based solution for the same.

// A memoization based recursive program to count numbers // with difference between odd and even digit sums as 1 #include<bits/stdc++.h> using namespace std; // A lookup table used for memoization. unsigned long long int lookup[50][1000][1000][2]; // Memnoization based recursive function to count numbers // with even and odd digit sum difference as 1. This function // conisders leading zero as a digit unsigned long long int countRec(int digits, int esum, int osum, bool isOdd, int n) { // Base Case if (digits == n) return (esum - osum == 1); // If current subproblem is already computed if (lookup[digits][esum][osum][isOdd] != -1) return lookup[digits][esum][osum][isOdd]; // Initialize result unsigned long long int ans = 0; // If current digit is odd, then add it to odd sum and recur if (isOdd) for (int i = 0; i <= 9; i++) ans += countRec(digits+1, esum, osum+i, false, n); else // Add to even sum and recur for (int i = 0; i <= 9; i++) ans += countRec(digits+1, esum+i, osum, true, n); // Store current result in lookup table and return the same return lookup[digits][esum][osum][isOdd] = ans; } // This is mainly a wrapper over countRec. It // explicitly handles leading digit and calls // countRec() for remaining digits. unsigned long long int finalCount(int n) { // Initialize number digits considered so far int digits = 0; // Initialize all entries of lookup table memset(lookup, -1, sizeof lookup); // Initializa final answer unsigned long long int ans = 0; // Initialize even and odd sums int esum = 0, osum = 0; // Explicitly handle first digit and call recursive function // countRec for remaining digits. Note that the first digit // is considered as even digit. for (int i = 1; i <= 9; i++) ans += countRec(digits+1, esum + i, osum, true, n); return ans; } // Driver program int main() { int n = 3; cout << "Coutn of "<<n << " digit numbers is " << finalCount(n); return 0; }

Output:

Count of 3 digit numbers is 54

Thanks to Gaurav Ahirwar for providing above solution.

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