Recursive string reversal function in javascript?

Question

I'm a pretty experienced frontend engineer with a weak CS background. I'm trying to get my head around the concept of recursion. Most of the examples and purported explanations I can find just aren't explaining it in a way I find easy to understand.

I set myself a task of writing a function that will reverse a string recursively. I know there has to be a base condition (i.e. the solution is found), but I can't figure out how to actually write something like this and could use a demo to study.

Could someone provide a sample function?

Answer 1

A 25% faster function: jsperf.com

function Reverse(str) {
  if (str === null) {
    return null;
  }
  if (str.length <= 1) {
    return str;
  }
  var first = str[0];
  var last = str[str.length - 1];
  var str1 = Reverse(str.substring(1, str.length - 1));
  return last + str1 + first;
}

var result = Reverse("a really serious string of nothingness making call stack to explode");

Answer 2

It is verbose, but I like making it easy to understand in logical steps:

function rev(soFar, count){
   console.log("asString: " + soFar );
   console.log("count: " + count);
   var len = soFar.length;
   var ret = soFar;//ret needs to be a reference to soFar
   if(len > count){
      var subd = soFar.substring(1,len);
      var first = soFar[0];
      //we want to inject the first letter at the index position one back from the length, minus what the count is at this point
      var indexOfInsert = len-1 - count;//so if count is 0 and length is 5, we want 4 (4 -0)
      var asArray = subd.split("");
      asArray.splice(indexOfInsert,0,first);
      count++;//need to increment count for the next round
      var asString = "";
    //recreate as string, not array - the default toString() makes this a comma delimited string. It is best toi just recreate it in a loop
    for(var i = 0; i<len; i++){
        asString+=asArray[i];
    }
    ret = rev(asString,count);//ret always needs to be reassigned
}
//only get here when count is greater than the length of the original string
return ret;//will always be a reference to soFar, which is being reassigned in the recursive loop

}

Then call it like:

var reversed = rev("Hello",0);
console.log("result",reversed);

Answer 3

So far the best I think:

function reverse(s) {
    if (s.length===1) return s;
    return reverse(s.slice(1)) + s[0];
}

Answer 4

Try this:

function recurse(s) {  
  if (s.length == 0) {  
    return '' // stopping condition  
  } else {  // return last char + result of function called with chars up to last char  
    return s.substring(s.length, s.length -1) + recurse(s.substring(0, s.length -1))  
  }
}  

Answer 5

One line of code using ternary operators you can easily reverse it.

Explanation: if string exists (if not null) then return recursion otherwise stop the recursion.

  function reverseString(str) {
    return (str ? reverseString(str.substring(1)) + str.charAt(0) : str);
  }

Function call:

console.log(reverseString('hello'));

Answer 6

This is a pretty straightforward C# implementation of the algorithm you asked for. I think it could be rewritten in javascript pretty easily.

/*
C#: The Complete Reference 
by Herbert Schildt 

Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/


// Display a string in reverse by using recursion. 

using System; 

class RevStr { 

  // Display a string backwards. 
  public void displayRev(string str) { 
    if(str.Length > 0)  
      displayRev(str.Substring(1, str.Length-1)); 
    else  
      return; 

    Console.Write(str[0]); 
  } 
} 

public class RevStrDemo { 
  public static void Main() {   
    string s = "this is a test"; 
    RevStr rsOb = new RevStr(); 

    Console.WriteLine("Original string: " + s); 

    Console.Write("Reversed string: "); 
    rsOb.displayRev(s); 

    Console.WriteLine(); 
  } 
}