问题
in C#, What is the Complexity of StringBuilder.ToString()? Is it O(1), O(N), or something else?
回答1:
It varies between framework version; in older versions StringBuilder works on a string directly, so there is no additional cost in .ToString(): it just hands you the data directly (which can mean oversized, but it makes it work); so O(1).
In newer framework version, it uses a char[] backing buffer, so now when you .ToString() it will probably need to copy 2 x Length bytes, making it O(N).
来源：https://stackoverflow.com/questions/24250487/what-is-the-complexity-of-stringbuilder-tostring