Growing with the Web

Reverse a string

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This article looks at the interview question - Reverse a string in the most efficient way possible. For example an input of "abc123" will result in the output "321cba".


This is a very common interview question, it tests whether a candidate understands how string concatenation and immutability works. A simple algorithm that does the job loops through each character and constructs a string in reverse order.

function reverse (text)
  define result ← ""
  foreach character c in text
    result ← c + result
  return result

The above algorithm is not very efficient though. This is because strings are immutable which means they cannot be modified after they are created. The algorithm creates a new string every iteration which takes worst case O(n), making the algorithm O(n^2).

The string can be reversed in O(n) time with a couple of different methods: using StringBuilder to construct the string as above, or by converting the string to a character array and back. The optimal solution largely depends on how strings are handled in the language that the function is being implemented in though.


public static String reverseViaCharArray(String text) {
    char[] charArray = text.toCharArray();
    int start = -1;
    int end = charArray.length;
    while (++start < --end) {
        char temp = charArray[start];
        charArray[start] = charArray[end];
        charArray[end] = temp;
    return String.valueOf(charArray);

public static String reverseViaStringBuilderA(String text) {
    StringBuilder sb = new StringBuilder();
    for (int i = text.length() - 1; i >= 0; i--) {
    return sb.toString();

public static String reverseViaStringBuilderB(String text) {
    return new StringBuilder(text).reverse().toString();
function reverseString(text) {
  var result = '';
  for (var i = text.length - 1; i >= 0; i--) {
    result += text[i];
  return result;


Here are two textbooks that provide many Q&A's like this post that I personally recommend, both were invaluable to me when I was studying for my interviews.


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