Time complexity of the code below

Question

I am trying to check the time complexity of the below simple program. The program replaces spaces in a string with '%20' .

1. The loop to count spaces (O(1) time)

     foreach (char k in s) { if (k == ' ') { spaces_cnt++; } } 

2. The loop to replace the spaces (O(n) where n is size of string)

     char[] c = new char[s.Length + spaces_cnt * 3]; int i = 0; int j = 0; while (i<s.Length) { if (s[i] != ' ') { c[j] = s[i]; j++; i++; } else { c[j] = '%'; c[j + 1] = '2'; c[j + 2] = '0'; j = j + 3; i++; } } 

So I am guessing it is a "O(n) + O(1)" solution. Please correct me if I am wrong.

Answer 1

The loop to count spaces takes O(n) , not O(1) , since you're iterating over – and performing a check on – each of the n characters in your string.

As you stated, the replacement loop takes O(n) . Two O(n) operations performed sequentially have a combined complexity of O(n) (constant factors are discarded in Big-O notation).

PS You know that you can achieve the equivalent of all your code using a single line?

s = s.Replace(" ", "%20");

Answer 2

Looks like you're trying to encode a string. If that's the case, you can use the UrlPathEncode () method. If you're only trying to encode spaces, use Replace() (as mentioned by Douglas).