LeetCode #5 Longest Palindromic Substring

LeetCode #5 Longest Palindromic Substring

Question

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example:

Input: "cbbd"
Output: "bb"

Solution

Approach #1

class Solution {
    func longestPalindrome(_ s: String) -> String {
        let arr = Array(s.utf16)
        var start = 0
        var end = 0
        for i in 0..<arr.count {
            expandAroundCenter(arr, left: i, right: i, start: &start, end: &end)
            expandAroundCenter(arr, left: i, right: i + 1, start: &start, end: &end)
            if end == arr.count - 1 { break }
        }
        let result = Array(arr[start...end])
        return String(utf16CodeUnits: result, count: result.count)
    }
    
    func expandAroundCenter(_ arr: [UTF16.CodeUnit], left: Int, right: Int, start: inout Int, end: inout Int) {
        if left < 0 || right >= arr.count || arr[left] != arr[right] { return }
        var l = left
        var r = right
        while l >= 0, r < arr.count, arr[l] == arr[r] {
            l -= 1
            r += 1
        }
        l += 1
        r -= 1
        if r - l > end - start {
            start = l
            end = r
        }
    }
}

Time complexity: O(n^2).

Space complexity: O(1).

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