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Replace Spaces With Characters

Last update: 10/26/2018 6:06:00 AM

Written by:Fatih KABAKCI

The question is to replace all spaces in a given string with a special characters. In this question, we are asked to not to use extra space to edit the string such as transferring characters to a new array or string. Use same string to solve the problem.

Hint:Since strings are immutable, we should use char arrays to edit characters without using an extra storage. However, we need to remember to give extra spaces in end of the string to be able to manipulate the characters by moving to the end.

The strategy is to start from end and move the characters through right in the char array. If there is a space encountered, then add special characters in reverse order.

package com.fatihkabakci.arrStr;

 * @author fkabakci 
 * Replace all spaces with the character %20. Use same char array. No extra space is used.
public class ReplaceSpaceWithCharacters {
	private static int getNumberOfSpaces(char[] str, int length) {
		int numberOfSpaces = 0;
		for(int i = 0; i < length; i++) {
			if(str[i] == " ") {
		return numberOfSpaces;
	public static void replaceAllSpaces(char[] str, int length) {
		int numberOfSpaces = getNumberOfSpaces(str, length);		
		int arrIndex = numberOfSpaces * 2 + length - 1;
		for(int i = length - 1; i >= 0 ; i--) {
			if(str[i] == " ") {
				str[arrIndex--] = "0";
				str[arrIndex--] = "2";
				str[arrIndex--] = "%";
			}else {
				str[arrIndex--] = str[i];
	public static void main(String[] args) {
		// extra space at the end of the string has to be !
		char[] input = "Fatih SF USA    ".toCharArray();
		replaceAllSpaces(input, 12);

The algorithm is O(n) complexity.

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