Having trouble re-indexing my array after looping from the end of the array while adding characters in place

Question

I am trying to solve this particular algorithm question:

You are given a license key represented as a string S which consists only alphanumeric character and dashes. The string is separated into N+1 groups by N dashes.

Given a number K, we would want to reformat the strings such that each group contains exactly K characters, except for the first group which could be shorter than K, but still must contain at least one character. Furthermore, there must be a dash inserted between two groups and all lowercase letters should be converted to uppercase.

Given a non-empty string S and a number K, format the string according to the rules described above.

Example 1: Input: S = "5F3Z-2e-9-w", K = 4

Output: "5F3Z-2E9W"

Explanation: The string S has been split into two parts, each part has 4 characters. Note that the two extra dashes are not needed and can be removed. Example 2: Input: S = "2-5g-3-J", K = 2

Output: "2-5G-3J"

Explanation: The string S has been split into three parts, each part has 2 characters except the first part as it could be shorter as mentioned above. Note: The length of string S will not exceed 12,000, and K is a positive integer. String S consists only of alphanumerical characters (a-z and/or A-Z and/or 0-9) and dashes(-). String S is non-empty.

I have written the following code:

const licenseKeyFormatting = (S, K) => {
    //convert to array, remove special characters, and capitalize
    let s = [...S.replace(/\W/g, '').toUpperCase()]
    let pos = 1
    //from end of array add '-' for every K
    for (let i = s.length - 1; i > 0; i--) {
        if (pos === K) {
            s.splice(i, 0, '-')
            pos = 1
            i-- //re-index bc adding to the array
        }
        pos++
    }
    return s
}


console.log(licenseKeyFormatting("5F3Z-2e-9-w", 4)) //5F3Z-2E9W
console.log(licenseKeyFormatting("2-5g-3-J", 2)) //2-5G-3J
console.log(licenseKeyFormatting("a-a-a-a-", 1)) // this test case fails should be A-A-A-A, I am getting AAA-A

I am pretty sure the flaw in my logic is due to the re-index, but I can't figure out how to address it.

Answer 1

You should not alter the index. Using splice to insert an element pushes the other elements back, however since you iterate from back to front that doesn't matter. You've already handled the shifted elements.

Another issue is setting pos = 1 in the loop. This is directly followed by pos++. So when pos reaches K the value of pos will be reset to 2 at the end of the loop. Either set pos = 0 (in the loop) so it ends on 1 or move pos++ into the else section.

const licenseKeyFormatting = (S, K) => {
    //convert to array, remove special characters, and capitalize
    let s = [...S.replace(/\W/g, '').toUpperCase()]
    let pos = 1
    //from end of array add '-' for every K
    for (let i = s.length - 1; i > 0; i--) {
        if (pos === K) {
            s.splice(i, 0, '-')
            pos = 0
        }
        pos++
    }
    return s.join("") // <- added join for cleaner console output
}


console.log(licenseKeyFormatting("5F3Z-2e-9-w", 4)) //5F3Z-2E9W
console.log(licenseKeyFormatting("2-5g-3-J", 2)) //2-5G-3J
console.log(licenseKeyFormatting("a-a-a-a-", 1)) // this test case fails should be A-A-A-A, I am getting AAA-A