Length of String Compression

Let's define the "compacting" of a string as taking an input like the following: 'sstttrrrr'.

We want to take the number of sequential appearances (number of times it shows up in a row) of each letter:

s: 2
t: 3
r: 4

And denote them next to the original character in the string, getting rid of the duplicates. In our above example, our output is: 's2t3r4'.

How long is this compacted string? Could you write a method that takes in a string and returns the length of the compacted one? Could you solve it using only O(1) extra space?

Here are few other examples:

compactLength('aabbbbbbbbbbbbb')
// 5 because we end up with `a2b13`

Another to consider:

compactLength('s')
// 1 because we still end up with just `s`

Constraints

• Length of the given string <= 100000
• The string comprises of ASCII characters
• Expected time complexity : O(n)
• Expected space complexity : O(1)