AlgoDaily 03: Reverse Only Alphabetical


“You are given a string that contains alphabetical characters (a - z, A - Z) and some other characters ($, !, etc.). For example, one input may be:


Can you reverse only the alphabetical ones?”

First thought is to use a stack for this: go through the string forwards, pushing only letters on to the stack. Then go through the positions again, popping the stack for a letter position but leaving non-letters alone:

function reverseOnlyAlphabetical(input) {
	let letterStack = [...input].filter(x => /[a-zA-Z]/.test(x))
	return [...input].map((x) => {
		return /[a-zA-Z]/.test(x) ? letterStack.pop() : x;

Test cases:

// "sll!$ehaes3"

//  "1adj90sak3"

This seems different to the solution given on AlgoDaily, which I actually found hard to follow. The solution there is to get the whole string reversed, then go through swapping the letters for their counterparts in the reversed string, in place.

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