URLify

Write a function that replaces all spaces in a String with a "%20". You have to assume that the String is long enough to contain the additional characters it needs. Within this function you will also be given the "true length" - this means the length of the string without the additional spaces.

For example if we give you:

String:

String s = "Mr John Smith    ";
int TrueLength = 13;

It should return:

"Mr%20John%20Smith"

👉 Link here to the repo where you can solve the problem

👌 Tips

Many times it is easier to modify a String iterating with a loop from the end to the beginning

You may need to know in advance the number of spaces you need, could you simply count them?

👊 Solution 1

A very common and efficient way to edit Strings is to edit it starting at the end. This is quite useful since we have a certain “buffer” at the end that allows us to edit without worrying about what we are writing about.

In this solution we will use two "scans" in the first we will count the spaces that we have in the "trueLength", if we multiply this by 2 and add it to trueLength we will have the total length of the String already with the "% 20" added.

In the second scan (which will be done backwards) when we find a space we will add a “02%”, and if we find a character we simply copy it.

public void solution(char[] str, Integer trueLength) {
  int spaceCount = 0, index;

  for (int i = 0; i < trueLength; i++) {
    if(str[i] == ' ') spaceCount++;
  }

  index = trueLength + (spaceCount * 2);
  if (trueLength < str.length) str[trueLength] = '\0'; // end array

  for (int i = trueLength - 1; i >= 0; i--) {
    if (str[I] == ' ') {
      str[index - 1] = '0';
      str[index - 2] = '2';
      str[index - 3] = '%';
      index = index - 3;
    } else {
      str[index - 1] = str[i];
      index--;
    }
  }
}

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Borrowed question from the book “Cracking the coding interview”