Replace all spaces in a string with %20

October 01, 2020

Problem Statement

Replace all spaces in a string with ‘%20’ (three characters). Do this in place and assume you have enough space in array

Solution (Simple)

This is the simplest solution which many people might think.

  • Iterate from left side (begining)
  • After encountering a space, first shift entire array by 2 places.
  • Put %20 at consecutive location
  • Repeat this process

Obviously, if you see you need to shift the remaining array as many times as number of spaces. And, is not an efficient algorithm.

I will not put this silly code.


The complexity would be close to O(n^2)

A Better Solution

If you start from left side, you saw we need to shift the array. What if we start from the right side.

  • First calculate number of spaces in array, this will help in calculating the resultant last index
  • Start iterating from right most side.
  • Maintain two index counters. One for original array characters, other for resultant index positions.
  • Start copying character one by one.
  • If found space, just copy consecutive %20

At the end, you will achieve the purpose. Lets see the code

private static int countSpaces(char[] input) {
   int c = 0;
   for (int i=0; i<input.length; i++) {
      if (input[i] == ' ') {
         c ++;
   return c;

public static int replace(char[] input, int len) {
   int spaces = countSpaces(input);
   if (spaces == 0) return len;
   int lastIndex = (len-1) + (2 * spaces);
   int resultIndex = lastIndex;
   for (int i=len-1; i>=0; i--) {
      if (input[i] == ' ') {
         //copy %20
         input[lastIndex] = '0';
         input[lastIndex-1] = '2';
         input[lastIndex-2] = '%';
         lastIndex -= 3;
      else {
         //copy actual characters
         input[lastIndex] = input[i];
         lastIndex --;
   return resultIndex;


The complexity is O(n)

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