Multiply By 11

Given a positive number as a string, multiply the number by 11 and also return it as a string. However, there is a catch:

You are NOT ALLOWED to simply cast the numeric string into an integer!

Now, how is this challenge even possible? Despite this, there is still a way to solve it, and it involves thinking about how someone might multiply by 11 in their head. See the tips below for guidance.

Examples

multiplyBy11("11") ➞ "121"

multiplyBy11("111111111") ➞ "1222222221"

multiplyBy11("1213200020") ➞ "13345200220"

multiplyBy11("1217197941") ➞ "13389177351"

multiplyBy11("9473745364784876253253263723") ➞ "104211199012633638785785900953"

Tip

There is a simple trick to multiplying any two-digit number by 11 in your head:

1. Add the two digits together
2. Place the sum between the two digits!
3. Note if the total goes over, carry the sum on to the next digit

// 23 * 11
// Add together 2 and 3 to make 5
// Place 5 between the two digits to make 253

// 77 * 11
// Add together 7 and 7 to make 14
// Place 4 between the two digits to make 747
// Carry the 1 to make 847

Notes

• Check out this resource to find out how this process can be utilized for numbers of any length.
• This challenge was greatly inspired by Mubashir Hassan's challenge on adding two numbers together.
• A recursive version of this challenge can be found via this link.