[PYTHON] A good way to make a recursive function that reverses the characters

Concept for displaying characters in reverse order

1. Get the last value of a particular word
2. Delete the last value of an existing word
3. Get the last value of the word after 2 and connect it after the obtained character of 1.
4. Repeat steps 1 to 3 until the number of characters in the word is full.
5. Output the acquired characters

Completion code

test.php

<?php
    function reverseStr($word,$reword){
        if(strlen($word)<1){
            return $reword;
        }
        return reverseStr(substr($word,0,-1), $reword.$word[-1]);
    }
    $word = "abcdefgh";
    echo reverseStr($word, null);
    //hgfedcba
    echo strlen("");
    //0
?>

Code description

There are various types of recursive functions, but they are created with tail recursion in mind. This will be explained while comparing the differences with ordinary recursive functions.

For tail recursion

__ Prerequisite: Input value is abcd __

test.php

<?php
    function a($word,$reword){
        if(strlen($word)<1){
            return $reword;
        }
        return a(substr($word,0,-1), $reword.$word[-1]);
    }
?>

Commentary

The transition of the function assumed when the initial input value is (abcd, "") is as follows. a(abcd,"") => a(abc,d) => a(ab,dc) => a(a,dcb) => a("",dcba) => dcba It can be said that the load on the memory is small because the values ​​are returned one by one and the processing is executed.

For normal recursion

__ Prerequisite: Input value is abcd __

test.php

<?php
   function a($word){
        if(strlen($word)<1){
            return $word;
        }
        return $word[-1] + a(substr($word,0,-1));
    }
?>

Commentary

The transition of the function assumed when the initial input value is (abcd, "") is as follows. a(abcd) => a(abcd){ d + a(abc)　} => a(abcd){ d + a(abc) { c + a(ab) } } => a(abcd){ d + a(abc) { c + a(ab) { b + a(a) } } } => a(abcd){ d + a(abc) { c + a(ab) { b + a } } } => a(abcd){ d + a(abc) { c + ba } } => a(abcd){ d + cba } =>dcba

The commentary is currently being revised.

Codes in other languages ​​(for reference)

test.js

  function reverseStr(word, reword){
      if(word.length < 1){
          return reword;
      }
      return reverseStr(word.slice(0,-1), reword + word.slice(-1));
  }
  var word = "abcdefgh";
  console.log(reverseStr(word, ""));

test.rb

def reverseStr(word, reword)
   if(word.length < 1)
       return reword
   end
   return reverseStr(word[0..-2],reword+word[-1])
end
word = "abcdefgh"
puts reverseStr(word, "")

test.py

def reveseStr(word, reword):
    if len(word) < 1:
        return reword
    return reveseStr(word[0:-1], reword + word[-1])
    
word = "abcdefgh"
print(reveseStr(word, ""))