[Swift]LeetCode214. 最短回文串 | Shortest Palindrome

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Given a string s, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation.

Example 1:

Input: "aacecaaa"
Output: "aaacecaaa"

Example 2:

Input: "abcd"
Output: "dcbabcd"

---

给定一个字符串 s，你可以通过在字符串前面添加字符将其转换为回文串。找到并返回可以用这种方式转换的最短回文串。

示例 1:

输入: "aacecaaa"
输出: "aaacecaaa"

示例 2:

输入: "abcd"
输出: "dcbabcd"

---

52ms

class Solution {
    func shortestPalindrome(_ s: String) -> String {
        let characters = Array(s)
        var end = s.count
        while true {
            var j = 0
            var i = end-1
            while i >= 0 {
                if characters[i] == characters[j] {
                    j += 1
                }
                i -= 1
            }
            if j == end {
                break
            }
            end = j
        }
        return (s.suffix(s.count-end).reversed() + s)
    }
}

---

60ms

class Solution {
    func shortestPalindrome(_ s: String) -> String {
        let sArray = Array(s)
        var j = 0
        for i in (0..<s.count).reversed() {
            if sArray[i] == sArray[j] {
                j += 1
            }
        }
        if j == s.count { return s }
        let suffix = String(s[s.index(s.startIndex, offsetBy: j)...])
        let prefix = suffix.reversed()
        let mid = shortestPalindrome(String(s[s.index(s.startIndex, offsetBy:0)..<s.index(s.startIndex, offsetBy:j)]))
        
        return prefix + mid + suffix
    }
}

---

68ms

class Solution {
    func shortestPalindrome(_ s: String) -> String {
        return String(checkShortest(Array(s)))
    }
    
    func checkShortest(_ s: [Character]) -> [Character] {
        if s.count <= 1 {
            return s
        }
        
        var j = 0
        for i in stride(from: s.count - 1, through: 0, by: -1) {
            if s[s.index(s.startIndex, offsetBy: i)] == s[s.index(s.startIndex, offsetBy: j)] {
                j += 1
            }
        }
        
        if j == s.count {
            return s
        }
        
        let suffix = s[j..<s.count]
        let middle = checkShortest(Array(s[0..<j]))
        let prefix = suffix.reversed()
        return prefix + middle + suffix
    }
}

---

80ms

class Solution {
    func shortestPalindrome(_ s: String) -> String {
        let sArray = Array(s)
        var j = 0
        for i in (0..<sArray.count).reversed() {
            if sArray[i] == sArray[j] {
                j += 1
            }
        }
        if j == sArray.count { return s }
        let suffix = String(sArray[j...])
        let prefix = suffix.reversed()
        let mid = shortestPalindrome(String(sArray[0..<j]))
        return prefix + mid + suffix
    }
}

---

84ms

class Solution {
    func shortestPalindrome(_ s: String) -> String {
        if s.isEmpty {
            return s
        }
        
        var sArr = Array(s)
        
        if sArr == sArr.reversed() {
            return s
        }
        
        var mArr = [Character]()
        for i in 0..<sArr.count {
            mArr.append("#")
            mArr.append(sArr[i])
        }
        mArr.append("#")
        var counts = Array(repeating: 0, count: mArr.count)
        var right = 1
        var center = 1
        var left = sArr.count - 1
        for i in 1..<mArr.count / 2 {
            let minx = right <= i ? 1 : min(right - i, counts[center * 2 - i] + 1)
            for r in minx...i {
                if mArr[i-r] != mArr[i+r] {
                    break
                }
                counts[i] = r
                if i==r {
                    left = sArr.count - i
                }
            }
            
            if right < i + counts[i] {
                right = i + counts[i]
                center = i
            }
        }
        var res = [Character]()
        var k = sArr.count - 1
        for _ in 0..<left {
            res.append(sArr[k])
            k -= 1
        }
        res.append(contentsOf: sArr)
        
        return String(res)
    }
}

---

128ms

class Solution {
    func shortestPalindrome(_ s: String) -> String {
        let characters = Array(s)
        var end = s.count
        while true {
            var j = 0
            var i = end-1
            while i >= 0 {
                if characters[i] == characters[j] {
                    j += 1
                }
                i -= 1
            }
            if j == end {
                break
            }
            end = j
        }
        let prefix = String(s.suffix(s.count-end).reversed())
        return (prefix + s)
    }
}

---

3008ms

class Solution {
    func shortestPalindrome(_ s: String) -> String {
        var i:Int = 0
        var end:Int = s.count - 1
        var j:Int = end
        var arr:[Character] = [Character]()
        for char in s.characters
        {
            arr.append(char)
        }
        while (i < j)
        {
            if arr[i] == arr[j]
            {
                i += 1
                j -= 1
            }
            else
            {
                i = 0
                end -= 1
                j = end
            }
        }
       let res:String = s.subString(end + 1).reversed() + s
       return res
    }
}

extension String
{
    // 截取字符串：从index到结束处
    // - Parameter index: 开始索引
    // - Returns: 子字符串
    func subString(_ index: Int) -> String {
        let theIndex = self.index(self.endIndex, offsetBy: index - self.count)
        return String(self[theIndex..<endIndex])
    }
}

---

4052ms

class Solution {
    func shortestPalindrome(_ s: String) -> String {
        let characters = Array(s)
        let count = s.count
        var end = count-1
        while end >= 1 {
            if !isPalindrome(arr: characters, end: end) {
                end -= 1
            } else {
                break
            }
        }
        // let prefix = String(s.suffix(count-1-end).reversed())
        return (s.suffix(count-1-end).reversed() + s)
    }
    
    func isPalindrome(arr: [Character], end: Int) -> Bool {
        for i in 0 ... end/2 {
            let endIndex = end-i
            if arr[i] != arr[endIndex] {
                return false
            }
        }
        return true
    }
}