String Compression

Given a string chars, compress each maximal run of equal consecutive characters.
Write the character once.
If the run length is greater than one, immediately follow it with the decimal digits of that length.
Return the complete compressed string. Run counts may contain multiple digits.

Function
compress(chars: String) → String

Examples
Example 1
chars = "aabbccc"
return = "a2b2c3"
The consecutive runs have lengths 2, 2, and 3.
Example 2
chars = "a"
return = "a"
A run of length one has no numeric suffix.
Example 3
chars = "abbbbbbbbbbbb"
return = "ab12"
The first run has length one, and the twelve consecutive b characters use the two digits 12.

Constraints
1 <= chars.length <= 100000.
chars contains printable ASCII characters other than line breaks.

The trick is a single pass with two pointers. Set i at the start of a run, move j forward while chars[j] equals chars[i], then compute the run length as j - i. Append chars[i] to a builder. If the length is greater than one, append str(length), which handles 12, 100, or 100000 automatically. Then set i = j and repeat. Time is O(n), and with n up to 100000 you must use a list or string builder, not repeated string concatenation in a loop. Common pitfalls: appending the count for runs of length one, forgetting to flush the final run, and writing digits one at a time as if counts were a single character. Also remember the input can include digits as characters, which is fine since you return a new string and never decode. If you blank on the loop structure during the live OA, StealthCoder gives you a clean reference implementation to check against.

If this hits your live OA and you blank, StealthCoder solves it in seconds, invisible to the proctor.