Actaully, the correct answer to explaining why is : "System.String implements IEnumerable"
The fact that a string may (or may not) be inernally implemented as an array is completely irrelevant. It's also important to note that, by it's public interface, a String is NOT "an array of characters". If you, for example, tried calling Array.Fill()" on a string, it would fail.

strings are an IEnumerable. The ToCharArray() is unnecessary.

What makes you think the Replace() would leave spaces on the start or end?

It's not redundant, just bad form. input[i] is a char, " " is a string. The line should be "if (input[i] == ' ')" or better "if (Char.IsWhiteSpace(input[i]))"

Nice to preallocate the StringBuilder, but why are you dragging floating point into the calculation? The result has to be an integer, so you might as well, do everything in integer math.

Tsk,Tsk. You always do the entire calculation twice. sloppy & wasteful.

then make one lol
its called community effort...