ReverseString, a C# interview-question

Question

I had an interview question that asked me for my \'feedback\' on a piece of code a junior programmer wrote. They hinted there may be a problem and said it will be used heavily o

Answer 1

Since strings are immutable, each += statement will create a new string by copying the string in the last step, along with the single character to form a new string. Effectively, this will be an O(n²) algorithm instead of O(n).

A faster way would be (O(n)):

// pseudocode:
static string ReverseString(string input) {
    char[] buf = new char[input.Length];
    for(int i = 0; i < buf.Length; ++i)
       buf[i] = input[input.Length - i - 1];
    return new string(buf);
}