One Pager Cheat Sheet

• You can reverseOnlyAlphabetical characters in a given string with length <= 100000 and composed of ASCII characters in O(n) time and space complexity.
• The two pointers with swap method is recommended to reverse a regular string.
• We can reverseArray the alphabetical characters of a string using /[a-zA-Z]/ in a for loop to efficiently reverse only the alphabetical characters.
• We can swap out alphabetical characters in the string with elements from our newly created reversedAlpha based on the position/index.
• Using the .map and .filter methods, one can reverse only alphabetical characters in a more succinct manner.
• We split the string into an array and loop over it with map to return non-alphanumeric characters in their current positions.
• We splice and join our array to remove unmatched characters and turn it back into a string, resulting in an O(n) time and space complexity.

This is our final solution.

To visualize the solution and step through the below code, click Visualize the Solution on the right-side menu or the VISUALIZE button in Interactive Mode.

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}

var assert = require('assert');

​

function reverseOnlyAlphabetical(str) {

  const alphaChars = [];

  str = str.split(''); // strings are immutable in JS

​

  for (let char of str) {

    if (/[a-zA-Z]/.test(char)) {

      alphaChars.push(char);

    }

  }

​

  const reversedAlpha = reverseArray(alphaChars);

​

  let idxRA = 0;

  for (let i = 0; i < str.length; i++) {

    if (/[a-zA-Z]/.test(str[i])) {

      str[i] = reversedAlpha[idxRA++];

    }

  }

​

  return str.join('');

}

​

function reverseArray(arr) {

  let start = 0;

  let end = arr.length - 1;

​

  while (start <= end) {

    const temp = arr[start];

    arr[start] = arr[end];

    arr[end] = temp;

Alright, well done! Try another walk-through.

If you had any problems with this tutorial, check out the main forum thread here.