Given a non-negative integer N, repeatedly add all its digits until the result has only one digit.
Example 1:
Input:
N = 98
Output:
8
Explanation:
Sum of digit of N = 98 is 9 + 8 = 17
Sum of digit of 17 = 1 + 7 = 8
Since 8 is the single digit number hence it is the
required answer.
Example 2:
Input:
N = 1
Output:
1
Explanation:
Since 1 is the single-digit number hence it is the
required answer.
Your Task:
You don't need to read input or print anything. Your task is to complete the function singleDigit() which takes an integer N as an input parameter and return the single-digit you will get after repeatedly adding the digits of N.
Expected Time Complexity: O(1)
Expected Auxiliary Space: O(1)
Constraints:
1 <= N <= 10^{18}

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