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tommy-3

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Learning Algorithms with JS, Python and Java 1: String Reversal

When I study natural languages, I've always enjoyed comparing them. It was intriguing to know, for example, that the English and German prefix "un-", the Latin prefix "in-" and the Greek prefix "ἀ-" are all thought to stem from the same Proto-Indo-European prefix "*n̥-". As a (semi-)beginner of programming, I thought it might be also fun to try to learn them in a comparative manner.

I'll follow Stephen Grider's Udemy course The Coding Interview Bootcamp: Algorithms + Data Structures, which is completely in JavaScript, and solve the same question in JavaScript, Python and Java, which are just three languages I happen to know the basics of.

The first task in the course is to reverse the given string. Stephen introduces three JavaScript answers.

1. Simple Solution

JavaScript:

``````function reverse(str) {
return str.split('').reverse().join('');
}
``````

Its translation into Python would be:

``````def reverse(str):
chars = [char for char in str]
chars.reverse()
return ''.join(chars)
``````

In idiomatic Python:

``````def reverse(str):
return str[::-1]
``````

The Java equivalent of the first JS code would be:

``````import org.apache.commons.lang3.ArrayUtils;

static String reverse(String str) {
char[] chars = str.toCharArray();
ArrayUtils.reverse(chars);
return String.valueOf(chars);
}
``````

The standard Java way to achieve it seems to be:

``````static String reverse(String str) {
return new StringBuilder(str).reverse().toString();
}
``````

2. Using a For Loop

JavaScript:

``````function reverse(str) {
let reversed = '';

for (let char of str) {
reversed = char + reversed;
}

return reversed;
}
``````

Python:

``````def reverse(str):
reversed = ''

for char in str:
reversed = char + reversed

return reversed
``````

Java:

``````static String reverse(String str) {
StringBuilder reversed = new StringBuilder();

for (char character : str.toCharArray()) {
reversed.insert(0, character);
}

return reversed.toString();
}
``````

which is actually very slow when a long string is given, because it searches for the 0th position every time.

This one works fast enough:

``````static String reverse(String str) {
StringBuilder reversed = new StringBuilder();

for (int i = str.length() - 1; i >= 0; i--) {
reversed.append(str.charAt(i));
}

return reversed.toString();
}
``````

3. Using the Reduce Method

This is not very practical or efficient, but just to explore what the languages offer.

JavaScript:

``````function reverse(str) {
return str.split('').reduce((reversed, char) => char + reversed, '');
}
``````

Python:

``````from functools import reduce

def reverse(str):
return reduce(lambda reversed, char: char + reversed, str)
``````

Java:

``````static String reverse6(String str) {
return str.chars()
.mapToObj(c -> String.valueOf((char) c))
.reduce("", (reversed, chr) -> chr + reversed);
}
``````

I hope someone would find this interesting or helpful and I'd appreciate any suggestion or comment.

ZeroKoriel

Hi tommy-3, your post is funny. It reminded me of various tasks at the university.

Congratulations! Keep going!

tommy-3