Longest Palindromic Substring in Swift

Longest Palindromic

Substring in Swift

Welcome to the Longest Palindromic Substring in Swift page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

import Foundation

let usage = """
    Usage: please provide a string that contains at least one palindrome
    """

extension Collection {
    subscript(safe index: Index) -> Element? {
        indices.contains(index) ? self[index] : nil
    }
}

func findLongestPalindrome(_ s: String) -> String {
    let originalChars = Array(s)
    guard !originalChars.isEmpty else { return "" }

    let processed: [Character] = ["#"] + originalChars.flatMap { [$0, "#"] }

    let n = processed.count
    var radii = Array(repeating: 0, count: n)
    var center = 0
    var rightBoundary = 0

    var maxRadius = 0
    var centerOfMax = 0

    for i in 0..<n {
        if i < rightBoundary {
            let mirror = 2 * center - i
            radii[i] = min(rightBoundary - i, radii[mirror])
        }

        var left = i - (1 + radii[i])
        var right = i + (1 + radii[i])

        while left >= 0 && right < n, processed[left] == processed[right] {
            radii[i] += 1
            left -= 1
            right += 1
        }

        if i + radii[i] > rightBoundary {
            center = i
            rightBoundary = i + radii[i]
        }

        if radii[i] > maxRadius {
            maxRadius = radii[i]
            centerOfMax = i
        }
    }

    guard maxRadius > 1 else { return "" }

    let start = (centerOfMax - maxRadius) / 2
    let resultRange = start..<(start + maxRadius)

    return String(originalChars[resultRange])
}

guard let input = CommandLine.arguments[safe: 1], !input.isEmpty else {
    print(usage)
    exit(1)
}

let result = findLongestPalindrome(input)

if result.isEmpty {
    print(usage)
    exit(1)
} else {
    print(result)
}

Longest Palindromic Substring in Swift was written by:

• Ștefan-Iulian Alecu

If you see anything you'd like to change or update, please consider contributing.

How to Implement the Solution

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How to Run the Solution

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