Max of Three with a Helper in Swift

Given three integers a, b and c, return the largest of them.

This time the work is split between two functions. The first one is a helper: it gets two numbers and returns the larger one. The second one, the function that is graded, finds the largest of the three numbers by calling the helper, so it never compares numbers itself.

Don't use a built-in max function: the helper does the comparing. The numbers can be negative, and some of them can be equal.

Example 1:

Input: a = 3, b = 7, c = 5
Output: 7
Explanation: The larger of 3 and 7 is 7, and the larger of 7 and 5 is still 7.

Example 2:

Input: a = -3, b = -8, c = -1
Output: -1
Explanation: All three numbers are negative. -1 is the closest to zero, so it is the largest.

Example 3:

Input: a = 4, b = 4, c = 2
Output: 4
Explanation: a and b tie for the largest. Either one is the answer, because the value is the same: 4.

Constraints:

• a, b and c are integers between -1000000 and 1000000.
• Two or all three of them can be equal.