5. Longest Palindromic Substring

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

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

Input: "cbbd"
Output: "bb"

答案：

    func longestPalindrome(_ s: String) -> String {
        if s.characters.count <= 1 {
            return s
        }
       
        let charArr = Array(s.characters)
        let strLen = s.characters.count
    
        var start = 0
        var lenth = 0
        var tmpStart = 0
        var tmpLen = 0
        
        for j in 0...500 {
            if i-j >= 0 && i+1+j < strLen && charArr[i-j] == charArr[i+1+j] {
                tmpStart = i - j
                tmpLen = (j+1) * 2;
            } else {
                break;
            }
            
            if tmpLen > lenth {
                lenth = tmpLen
                start = tmpStart
                tmpLen = 0
                tmpStart = 0
            }
    
            for j in 0...500 {
            if i-j >= 0 && i+j < strLen && charArr[i-j] == charArr[i+j]{
                tmpStart = i - j
                tmpLen = j*2 + 1;
            } else {
                break;
            }
            
            if tmpLen > lenth {
                lenth = tmpLen
                start = tmpStart
                tmpLen = 0
                tmpStart = 0
            }
        }
        
        let startIndex = s.index(s.startIndex, offsetBy: start)
        let endIndex = s.index(s.startIndex, offsetBy: start + lenth)
        let range = startIndex..<endIndex
        
        return s.substring(with: range)
    }