Posted on Apr 19, 2023

Day 6. Reverse String problems (two pointers)

We cannot directly swap characters and change in-place with Javascript string, just like we did for array of characters.
- why? Because String is immutable
We cannot directly assign a letter via index to an empty string. you need to use + operation to write on the string.

First Attempt Code

// string => array => swap => string var reverseWords = function(s) { let words = s.split(" "); for(let i = 0; i < words.length; i++) { words[i] = swap(words[i]); } return words.join(" "); }; var swap = function(word) { let chars = word.split(''); let leftP = 0; let rightP = chars.length - 1; while(leftP < rightP) { if(chars[leftP] !== chars[rightP]) { let leftChar = chars[leftP]; chars[leftP] = chars[rightP]; chars[rightP] = leftChar; } rightP--; leftP++; } return chars.join(''); }

Improved version

/** * @param {string} s * @return {string} */ // improved version of swap function above function reverseString(s){ let arr = s.split(''); let length = ~~(arr.length/2); for(let i = 0; i< length; i++){ [arr[i],arr[s.length-1-i]] = [arr[arr.length-1-i],arr[i]]; } return arr.join(''); } var reverseWords = function(s) { let arr = s.split(' '); for(let i = 0; i<arr.length; i++){ arr[i] = reverseString(arr[i]); } return arr.join(' ') };

Second Way

I was trying to think of a way to stick with two pointers, but at the end, it doesn't seem to different.
This is not too different from the first way, except that here I didn't separate the works. From clean code perspective, it would be better to take out content of while loop as separate function outside.
Maybe in javascript, first way could be better or similar to this second method.

var reverseWords = function(s) { let wordStart = 0; let wordEnd = 1; let newWords = []; while(wordEnd <= s.length) { // hits the space or end if(wordEnd === s.length || s[wordEnd] == ' ') { // extract word and reverse let word = s.substring(wordStart, wordEnd); let reverseWord = ""; // add char from the end of substring for(let i = word.length - 1; i >= 0; i-- ) { reverseWord += word[i]; } newWords.push(reverseWord); // update wordStart to a beginning of next word wordStart = wordEnd + 1; } // increment wordEnd index until it hits the space or end wordEnd++; } // console.log(newWords); return newWords.join(" "); };

Python solution

just interesting

class Solution: def reverseWords(self, s: str) -> str: return " ".join([word[::-1] for word in s.split()])

~~ operator

In JavaScript, ~~ is a bitwise operator that performs a double bitwise NOT operation.

The double bitwise NOT operator converts its operand to a signed 32-bit integer. It is equivalent to Math.floor() for non-negative numbers and Math.ceil() for negative numbers.

For example, ~~3.14 returns 3, and ~~(-3.14) returns -3.

The ~~ operator can be used as a faster alternative to Math.floor() or parseInt() in certain situations where you only need to convert a positive number to an integer. However, it should be used with caution as it can produce unexpected results for negative numbers or non-numeric inputs.