How to Solve Digit Counts Problem?

Description

Count the number of k’s between 0 and n. k can be 0–9.

Example

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Step by Step Solution

The step 0 is to get each number from the input, we need to a simple loop.

The next step is to think through how to get the number of each number. We would like to do some experience.

Let’s consider the single-digit number (0–9) with given input k, it is very straightforward thought, just do a simple comparison.

Now let’s consider the “2+ digit” number, the way to extract each digit is to get the reminder with dividing by 10. For example, to extract each number 1,2,3 from the number 123.

We can do the calculation as 123 % 10 = 3; 12 % 10 = 2; 1 % 10 = 1. And after we extract each number, we utilize the same way we do the comparison with single-digit number.

For the above analysis, we could summarize the below steps:

1. Get the reminder with dividing by 10;

2. Do comparison;

3. Renew the number by dividing by 10;

4. Repeat the step 1–3, until the end condition, which is the renewed number is 0;

Now let’s write some pseudocode -

Let’s give it a try but the answer is wrong, let’s add some debug statement and understand what’s going on.

We found out that there is something weird in the line 20, the t is changed as the float value. That’s not we want. Since Javascript is the loose-type language, we would need to manually make it as integer using the Math.floor() operation to make it as integer.

So the final solution is

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