- The nth level result is from counting each digit of the result of (n-1) th level.
- null or empty string
- white spaces
- +/- sign
- calculate real value
- handle min & max
The below Solution 1 is the most popular solution we can find online.
But here is another way (Solution 2) to solve the problem with StringBuffer.
The Solution 2 is easier to understand and the code is cleaner.