public class Solution {
    public string ReplaceSpaces(string S, int length) {
        StringBuilder resultSB = new StringBuilder();

        for (int i=0; i<length; i++)
        {
            if (S[i] == ' ')
            {
                resultSB.Append("%20") ;
            }
            else resultSB.Append(S[i]);
        }

        return resultSB.ToString();
    }
}