count the number of spaces in the string, say this number is n allocate a new string whose length is the same + 2 * n maintain two indices, i and j, i starts at the start of the original string and j starts at the start of the new string while i is less than the length of the string: if symbol at i is a space: copy %20 to the new string at j, advancing j by 3 else copy symbol at i in string to symbol at j in new string, advancing j by 1 advance i by 1