Minimum count of numbers required ending with 7 to sum as a given number

• Difficulty Level : Medium
• Last Updated : 26 May, 2021

Given an integer N, the task is to find the minimum count of numbers ending with 7 such that the sum of these numbers is N.
Examples:

Input: N = 38
Output:
7 + 7 + 7 + 17

Input: N = 46
Output: -1
46 cannot be represented as the sum
of integers ending with 7.

Input: N = 215
Output:
7 + 7 + 7 + 7 + 187

Approach:

• First observation here is that every number greater than or equal to 70 can always be written as the sum of numbers all ending with 7. For example, for 82 the last digit is 2, so at least 6 numbers ending with 7 are required i.e. (7 * 6 = 42). An array hasharr[] can be created where hasharr[i] represents the minimum number of numbers required having the last digit as 7 so the resultant sum has the last digit as i.
• If the number is less than 70 then N has to be checked whether it is less than the sum of the minimum number of numbers ending with digit seven 7. If it is then it is not possible and print -1, otherwise if it is greater or equal than it is possible.

Below is the implementation of the above approach:

C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;` `const` `int` `TEN = 10;` `// Function to return the count of``// minimum numbers ending with 7``// required such that the sum``// of these numbers is n``int` `minCount(``int` `n)``{` `    ``// hasharr[i] will store the minimum``    ``// numbers ending with 7 so that it``    ``// sums to number ending with digit i``    ``int` `hasharr[TEN] = { 10, 3, 6, 9, 2, 5, 8, 1, 4, 7 };` `    ``// Its always possible to write numbers > 69``    ``// to write as numbers ending with 7``    ``if` `(n > 69)``        ``return` `hasharr[n % TEN];``    ``else` `{` `        ``// If the number is atleast equal to the``        ``// sum of minimum numbers ending with 7``        ``if` `(n >= hasharr[n % TEN] * 7)``            ``return` `(hasharr[n % TEN]);``        ``else``            ``return` `-1;``    ``}``}` `// Driver code``int` `main()``{``    ``int` `n = 38;` `    ``cout << minCount(n);` `    ``return` `0;``}`

Java

 `// Java implementation of the above approach``class` `GFG {``    ` `// Function to return the count of``// minimum numbers ending with 7``// required such that the sum``// of these numbers is n``static` `int` `minCount(``int` `n)``{``    ` `    ``// hasharr[i] will store the minimum``    ``// numbers ending with 7 so that it``    ``// sums to number ending with digit i``    ``int``[] hasharr = { ``10``, ``3``, ``6``, ``9``, ``2``,``                       ``5``, ``8``, ``1``, ``4``, ``7` `};` `    ``// Its always possible to write ``    ``// numbers > 69 to write as``    ``// numbers ending with 7``    ``if` `(n > ``69``)``        ``return` `hasharr[n % ``10``];``    ``else``    ``{``        ` `        ``// If the number is atleast equal``        ``// to the sum of minimum numbers``        ``// ending with 7``        ``if` `(n >= hasharr[n % ``10``] * ``7``)``            ``return` `(hasharr[n % ``10``]);``        ``else``            ``return` `-``1``;``    ``}``}` `// Driver code``public` `static` `void` `main (String[] args)``{``    ``int` `n = ``38``;``    ` `    ``System.out.println(minCount(n));``}``}` `// This code is contributed by spp____`

Python3

 `# Python3 implementation of the above approach` `# Function to return the count of``# minimum numbers ending with 7``# required such that the sum``# of these numbers is n``def` `minCount(n):``    ` `    ``# hasharr[i] will store the minimum``    ``# numbers ending with 7 so that it``    ``# sums to number ending with digit i``    ``hasharr ``=` `[ ``10``, ``3``, ``6``, ``9``, ``2``,``                 ``5``, ``8``, ``1``, ``4``, ``7` `]` `    ``# Its always possible to write ``    ``# numbers > 69 to write as``    ``# numbers ending with 7``    ``if` `(n > ``69``):``        ``return` `hasharr[n ``%` `10``]``    ``else``:``        ` `        ``# If the number is atleast equal``        ``# to the sum of minimum numbers``        ``# ending with 7``        ``if` `(n >``=` `hasharr[n ``%` `10``] ``*` `7``):``            ``return` `hasharr[n ``%` `10``]``        ``else``:``            ``return` `-``1` `# Driver code``n ``=` `38``;` `print``(minCount(n))` `# This code is contributed by spp____`

C#

 `// C# implementation of the above approach``using` `System;` `class` `GFG{``    ` `// Function to return the count of``// minimum numbers ending with 7``// required such that the sum``// of these numbers is n``static` `int` `minCount(``int` `n)``{``    ` `    ``// hasharr[i] will store the minimum``    ``// numbers ending with 7 so that it``    ``// sums to number ending with digit i``    ``int``[] hasharr = { 10, 3, 6, 9, 2,``                       ``5, 8, 1, 4, 7 };` `    ``// Its always possible to write``    ``// numbers > 69 to write as``    ``// numbers ending with 7``    ``if` `(n > 69)``        ``return` `hasharr[n % 10];``    ``else``    ``{` `        ``// If the number is atleast equal ``        ``// to the sum of minimum numbers``        ``// ending with 7``        ``if` `(n >= hasharr[n % 10] * 7)``            ``return` `(hasharr[n % 10]);``        ``else``            ``return` `-1;``    ``}``}` `// Driver code``public` `static` `void` `Main (String[] args)``{``    ``int` `n = 38;``    ` `    ``Console.WriteLine(minCount(n));``}``}` `// This code is contributed by spp____`

Javascript

 ``

Output:

`4`

Time Complexity: O(1)

Auxiliary Space: O(1)

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