[Swift]LeetCode5. 最长回文子串 | Longest Palindromic Substring
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➤微信公众号：山青咏芝（shanqingyongzhi）
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Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
Example 1:
Input: "babad" Output: "bab" Note: "aba" is also a valid answer.
Example 2:
Input: "cbbd" Output: "bb"
给定一个字符串 s，找到 s 中最长的回文子串。你可以假设 s 的最大长度为1000。
示例 1：
输入: "babad" 输出: "bab" 注意: "aba"也是一个有效答案。
示例 2：
输入: "cbbd" 输出: "bb"
40ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 guard s.count > 0 else{ 4 return "" 5 } 6 guard s.count > 1 else{ 7 return s 8 } 9 var str_arr: [Character] = ["#"] 10 for ele in Array(s){ 11 str_arr.append(ele) 12 str_arr.append("#") 13 } 14 // Array to record longest palindrome 15 var result_arr = [Int](repeating: 0, count: str_arr.count) 16 var center = 0, boundary = 0, maxLen = 0, result_center = 0 17 18 // 首位的 "#" 不用管 19 for i in 1..<str_arr.count-1{ 20 // calc mirror i = center-(i-center) 21 let iMirror = 2 * center - i 22 result_arr[i] = boundary > i ? min(boundary-i, result_arr[iMirror]) : 0 23 // Attempt to expand palindrome centered at i 24 while i-1-result_arr[i] >= 0 , i + 1 + result_arr[i] <= str_arr.count - 1, str_arr[i+1+result_arr[i]] == str_arr[i-1-result_arr[i]]{ 25 result_arr[i]+=1 26 } 27 // update center and boundary 28 // 用来记录的 29 if i + result_arr[i] > boundary{ 30 center = i 31 boundary = i+result_arr[i] 32 } 33 // update result 34 if result_arr[i] > maxLen{ 35 maxLen = result_arr[i] 36 result_center = i 37 } 38 } 39 let ans = String(s[s.s_index(offset: (result_center-maxLen)/2)..<s.s_index(offset: (result_center+maxLen)/2)]) 40 41 return ans 42 } 43 } 44 45 46 47 extension String{ 48 49 func s_index(offset: Int) -> String.Index{ 50 return self.index(self.startIndex, offsetBy: offset) 51 } 52 53 }
44ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 4 if s.count == 0 { 5 return "" 6 } 7 8 if s.count == 1 { 9 return s 10 } 11 12 // 字符串预处理 13 var ss = [Character]() 14 for c in s { 15 ss.append("#") 16 ss.append(c) 17 } 18 ss.append("#") 19 20 var p = Array<Int>(repeating: 0, count: s.count * 2 + 1) 21 var i = 1, mx = 0, center = 0, j = 0 22 var maxCenter = 0 23 while i < ss.count { 24 // 说明以i为中心时，已有匹配的回文字符子串 25 if mx - i > 0 { 26 // i关于对称中心id的对称点 27 j = 2 * center - i 28 p[i] = min(p[j], mx - i) 29 } else { 30 p[i] = 1 31 } 32 if i - p[i] >= 0 && i + p[i] < ss.count { 33 while ss[i - p[i]] == ss[i + p[i]] { 34 p[i] += 1 35 if i - p[i] < 0 || i + p[i] >= ss.count { 36 break 37 } 38 } 39 } 40 if p[i] + i > mx { 41 mx = p[i] + i 42 center = i 43 } 44 if p[maxCenter] < p[i] { 45 maxCenter = i 46 } 47 i += 1 48 } 49 let ret = ss[(maxCenter - p[maxCenter] + 1) ... (maxCenter + p[maxCenter] - 1)].filter { $0 != "#"} 50 return String(ret) 51 } 52 }
52ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 if s.count <= 1 { 4 return s; 5 } 6 7 // 1.间隔之间先插入# 8 var newString:String = "#"; 9 for var character in s { 10 newString.append(character); 11 newString = newString + "#"; 12 } 13 let characters = Array(newString); 14 15 // 2.遍历找出以每个节点作为轴最长半径 16 var maxId:Int = 0; 17 var max:Int = 0; 18 var ids:[Int] = []; 19 ids.append(1); 20 var maxLength:Int = 1; 21 var maxLengthIndex = 0; 22 23 for var i in 1...characters.count - 1 { 24 var j:Int = maxId - (i - maxId); 25 if max > i && j >= 0 { 26 ids.append(min(ids[j], max - i)); 27 }else{ 28 ids.append(1); 29 } 30 while i + ids[i] <= characters.count - 1 && i - ids[i] >= 0 && characters[i + ids[i]] == characters[i - ids[i]]{ 31 ids[i] += 1; 32 } 33 34 if i + ids[i] - 1 > max { 35 maxId = i; 36 max = i + ids[i] - 1; 37 } 38 39 if ids[i] > maxLength{ 40 maxLength = ids[i]; 41 maxLengthIndex = i; 42 } 43 } 44 let leftIndex = s.index(s.startIndex, offsetBy: (maxLengthIndex - (maxLength - 1))/2); 45 let rightIndex = s.index(leftIndex, offsetBy:maxLength - 1 - 1); 46 return String(s[leftIndex...rightIndex]); 47 } 48 }
60ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 guard s.count > 1 else { 4 return s 5 } 6 7 var resultLeftIndex: Int = 0 8 var maxLen: Int = 1; 9 10 var sArray = Array(s) 11 12 // 循环参数 13 var index = 0 14 15 while index < sArray.count { 16 var leftIndex = index 17 var rightIndex = index 18 19 // 找重复 20 while rightIndex + 1 < sArray.count && sArray[rightIndex] == sArray[rightIndex + 1] { 21 rightIndex = rightIndex + 1 22 } 23 24 // 下一次循环从重复数后开始继续找回文 25 index = rightIndex + 1 26 27 // 找以left~right为中心的回文 28 while leftIndex - 1 >= 0 && rightIndex + 1 < sArray.count && sArray[leftIndex - 1] == sArray[rightIndex + 1] { 29 leftIndex = leftIndex - 1 30 rightIndex = rightIndex + 1 31 } 32 33 // 判断本次回文的长度，如果最长那么记录它 34 let len: Int = rightIndex - leftIndex + 1 35 if len > maxLen { 36 resultLeftIndex = leftIndex 37 maxLen = len 38 } 39 } 40 41 let nss: NSString = NSString.init(string: s) 42 43 let result: String = nss.substring(with: NSMakeRange(resultLeftIndex, maxLen)) 44 return result 45 } 46 }
64ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 if s.count <= 1 { 4 return s; 5 } 6 7 // 1.间隔之间先插入# 8 var newString:String = "#"; 9 for var character in s { 10 newString.append(character); 11 newString = newString + "#"; 12 } 13 let characters = Array(newString); 14 15 // 2.遍历找出以每个节点作为轴最长半径 16 var maxId:Int = 0; 17 var max:Int = 0; 18 var ids:[Int] = []; 19 ids.append(1); 20 var maxLength:Int = 1; 21 var maxLengthIndex = 0; 22 23 for var i in 1...characters.count - 1 { 24 var j:Int = maxId - (i - maxId); 25 if max > i && j >= 0 { 26 ids.append(min(ids[j], max - i)); 27 }else{ 28 ids.append(1); 29 } 30 while i + ids[i] <= characters.count - 1 && i - ids[i] >= 0 && characters[i + ids[i]] == characters[i - ids[i]]{ 31 ids[i] += 1; 32 } 33 34 if i + ids[i] - 1 > max { 35 maxId = i; 36 max = i + ids[i] - 1; 37 } 38 39 if ids[i] > maxLength{ 40 maxLength = ids[i]; 41 maxLengthIndex = i; 42 } 43 } 44 let leftIndex = s.index(s.startIndex, offsetBy: (maxLengthIndex - (maxLength - 1))/2); 45 let rightIndex = s.index(leftIndex, offsetBy:maxLength - 1 - 1); 46 return String(s[leftIndex...rightIndex]); 47 } 48 }
76ms
1 extension String { 2 func getSubString(startIndex: Int, endIndex: Int) -> String { 3 let start = self.index(self.startIndex, offsetBy: startIndex) 4 let end = self.index(self.startIndex, offsetBy: endIndex) 5 return String(self[start...end]) 6 } 7 8 func isPalindrome() -> Bool { 9 guard self.count >= 1 else { 10 return true 11 } 12 var amount = 0 13 let chars = self.map { String($0) } 14 for i in 0..<chars.count / 2 { 15 if chars[i] == chars[chars.count - 1 - i] { 16 amount = amount + 1 17 } 18 } 19 return amount == chars.count / 2 20 } 21 } 22 23 class Solution { 24 func longestPalindrome(_ s: String) -> String { 25 if s.count <= 1 { 26 return s 27 } 28 var C = 0 29 var R = 0 30 31 let chars = s.map { String($0) } 32 var transformArr: [String] = [] 33 transformArr.append("^") 34 for char in chars { 35 transformArr.append("#") 36 transformArr.append(char) 37 } 38 transformArr.append("#") 39 transformArr.append("$") 40 41 var P = Array(repeating: 0, count: transformArr.count) 42 43 for i in 1..<transformArr.count - 1 { 44 let i_mirror = 2 * C - i 45 if R > i { 46 P[i] = min(R - i, P[i_mirror])// 防止超出 R 47 } else { 48 P[i] = 0;// 等于 R 的情况 49 } 50 51 while transformArr[i + 1 + P[i]] == transformArr[i - 1 - P[i]] { 52 P[i] = P[i] + 1 53 } 54 55 if i + P[i] > R { 56 C = i 57 R = i + P[i] 58 } 59 } 60 61 var maxLen = 0 62 var centerIndex = 0 63 for i in 1..<transformArr.count - 1 { 64 if P[i] > maxLen { 65 maxLen = P[i] 66 centerIndex = i 67 } 68 } 69 let start = (centerIndex - maxLen) / 2 //最开始讲的 70 71 return s.getSubString(startIndex: start, endIndex: start + maxLen - 1) 72 } 73 }
96ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 guard s.count > 1 else { 4 return s 5 } 6 7 var str = "" 8 9 for i in 0..<s.count { 10 let idx = String.Index(encodedOffset: i) 11 str.append(String(s[idx])) 12 str.append("#") 13 } 14 15 var p: [Int] = [1] 16 var ct = 0 // 中心位置 17 var mr = 1 // 回文字串的最大右边界 18 19 var chars = [Character](str) 20 21 22 for i in 1..<str.count { 23 let val = mr > i ? min(p[2 * ct - i], mr-i) : 1 // p[i]的值为其对称的值p[j]和右边界到i的值的最小值 24 p.append(val) 25 26 // 如果对称位置的字符相等则加1 27 while i-p[i] >= 0 28 && i+p[i] < chars.count 29 && chars[i-p[i]] == chars[i+p[i]] { 30 p[i] += 1 31 } 32 33 // 更新中心位置和最大右边界位置 34 if i + p[i] > mr { 35 mr = i + p[i] 36 ct = i 37 } 38 } 39 40 if let val = p.max() { 41 let index = p.index(of: val) 42 let start = String.Index(encodedOffset: index!-val+1) 43 let end = String.Index(encodedOffset: index!+val) 44 return String(str[start..<end]).replacingOccurrences(of: "#", with: "") 45 } 46 47 48 return "" 49 } 50 }
140ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 // Base case 4 guard s.count > 0 else { return "" } 5 6 // Convert String to Array for simplicity of use 7 let chars = Array(s) 8 9 var start = 0, end = 0 10 11 // Loop through 12 for i in 0..<chars.count { 13 let length = chars.expandFromCenter(i, i) 14 let length2 = chars.expandFromCenter(i, i + 1) 15 16 let resultedMax = max(length, length2) 17 18 if resultedMax > (end - start) { 19 start = i - (resultedMax - 1)/2 20 end = i + resultedMax/2 21 } 22 } 23 24 let result = String(chars[start...end]) 25 print(result) 26 27 // Return string 28 return result 29 } 30 } 31 32 extension Array where Array.Element: Equatable { 33 func expandFromCenter(_ lhs: Int, _ rhs: Int) -> Int { 34 var left = lhs, right = rhs 35 while left >= 0, right < count, self[left] == self[right] { 36 left -= 1 37 right += 1 38 } 39 40 return right - left - 1 41 } 42 }
152ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 let count = s.count 4 if count < 2{ 5 return s 6 } 7 8 var charArr = [Character]() 9 for char in s { 10 charArr.append(char) 11 } 12 13 var start = 0 14 var end = 0 15 for i in 0..<count { 16 var left = i 17 var right = i 18 19 while left >= 0 && right < count && charArr[left] == charArr[right] { 20 left -= 1 21 right += 1 22 } 23 let len1 = right - left - 1; 24 left = i 25 right = i + 1 26 while left >= 0 && right < count && charArr[left] == charArr[right] { 27 left -= 1 28 right += 1 29 } 30 let len2 = right - left - 1; 31 let len = max(len1,len2) 32 if len > end - start { 33 start = i - (len - 1) / 2 34 end = i + len / 2 35 } 36 } 37 if start == end { 38 return String(s[s.startIndex]) 39 } 40 41 return String(s[s.index(s.startIndex, offsetBy: start)...s.index(s.startIndex, offsetBy: end)]); 42 } 43 }
168ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 guard s.count > 0 else { 4 return "" 5 } 6 var s = Array(s) 7 var maxLength = 0, curLength = 0 8 var minLeft = 0, maxRight = 0 9 var i = 0 10 let count = s.count 11 while i < count { 12 var left = i, right = i 13 while left - 1 >= 0 && s[left - 1] == s[i] { 14 left -= 1 15 } 16 17 while right + 1 < count && s[right + 1] == s[i] { 18 right += 1 19 } 20 21 while left - 1 >= 0 && right + 1 < count && s[left - 1] == s[right + 1] { 22 left -= 1 23 right += 1 24 } 25 curLength = right - left + 1 26 if curLength > maxLength { 27 maxLength = curLength 28 minLeft = left 29 maxRight = right 30 } 31 i += 1 32 } 33 34 var result = "" 35 for j in minLeft...maxRight { 36 result += String(s[j]) 37 } 38 39 return result 40 } 41 }
188ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 if s == "" { 4 return ""; 5 } 6 var start = 0, end = 0 7 var store:[Character:[Int]] = [Character:[Int]]() 8 let lenght = s.count 9 let chars:[Character] = [Character] (s) 10 var result = "" 11 for i in 0..<lenght { 12 let length1 = testPalindrow(chars,i,i) 13 let length2 = testPalindrow(chars,i,i+1) 14 let length = max(length1, length2) 15 if (length > end - start) { 16 start = i - (length - 1) / 2 17 end = i + length/2 18 } 19 } 20 21 return String(chars[start...end]) 22 } 23 24 func testPalindrow (_ s:[Character], _ left:Int, _ right:Int) -> Int { 25 var count = s.count 26 var L = left 27 var R = right 28 while (L>=0 && R < count && s[L] == s[R]) { 29 L -= 1 30 R += 1 31 } 32 return R-L-1 33 } 34 }
192ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 guard s.count > 0 else{ 4 return "" 5 } 6 let str_arr = Array(s) 7 var start = 0, end = 0 8 for i in 0..<str_arr.count{ 9 let len_one = str_arr.expandAroundCenter(lhs: i, rhs: i) 10 let len_two = str_arr.expandAroundCenter(lhs: i, rhs: i+1) 11 let len = max(len_one, len_two) 12 if len > end - start { 13 start = i - (len - 1)/2 14 end = i + len/2 15 } 16 } 17 let result: String = String(str_arr[start...end]) 18 print(result) 19 return result 20 } 21 }
208ms
1 class Solution { 2 func longestPalindrome(_ s: String) -> String { 3 if s.count <= 1 { 4 return s 5 } 6 var stringArray: Array<Character> = Array(s) 7 var maxLength: Int = 0 8 var startIndex: Int = 0 9 // 2 * n - 1 10 for i in 0..<2 * stringArray.count - 1 { 11 var leftIndex: Int 12 var rightIndex: Int 13 leftIndex = i / 2 14 rightIndex = (i + 1) / 2 15 while stringArray[leftIndex] == stringArray[rightIndex] { 16 if rightIndex - leftIndex + 1 > maxLength { 17 maxLength = rightIndex - leftIndex + 1 18 startIndex = leftIndex 19 } 20 21 if leftIndex - 1 >= 0 && rightIndex + 1 < stringArray.count { 22 leftIndex -= 1 23 rightIndex += 1 24 } else { 25 break 26 } 27 } 28 } 29 return String(stringArray[startIndex..<startIndex + maxLength]) 30 } 31 }
2708ms：Manacher算法
1 class Solution { 2 //Manacher算法 3 func longestPalindrome(_ s: String) -> String { 4 if s == nil || s.count < 1 {return ""} 5 else if s.count == 1{return s} 6 var s = s 7 //注意，为了避免更新P的时候导致越界，我们在字符串T的前增加一个特殊字符， 8 //比如说‘$’,所以算法中字符串是从1开始的。、 9 var manaStr = "$#" 10 for i in 0..<s.count 11 { 12 //首先构造出新的字符串 13 manaStr += String(getChar(s,i)) 14 manaStr += "#" 15 } 16 //用一个辅助数组来记录最大的回文串长度，注意这里记录的是新串的长度，原串的长度要减去1 17 var rd:[Int] = [Int](repeating: 0,count: manaStr.count) 18 var pos = 0, mx = 0 19 var start = 0, maxLen = 0 20 for i in 1..<manaStr.count 21 { 22 rd[i] = i < mx ? min(rd[2 * pos - i], mx - i) : 1 23 //这里要注意数组越界的判断，源代码没有注意，release下没有报错 24 while(i + rd[i] < manaStr.count && i - rd[i] > 0 && getChar(manaStr,i + rd[i]) == getChar(manaStr,i - rd[i])) 25 { 26 rd[i] += 1 27 } 28 //如果新计算的最右侧端点大于mx,则更新pos和mx 29 if mx < i + rd[i] 30 { 31 pos = i 32 mx = i + rd[i] 33 } 34 if rd[i] - 1 > maxLen 35 { 36 start = (i - rd[i]) / 2 37 maxLen = rd[i] - 1 38 } 39 } 40 let index3 = s.index(s.startIndex,offsetBy:start) 41 let index4 = s.index(index3,offsetBy:maxLen - 1) 42 return String(s[index3...index4]) 43 } 44 45 func getChar(_ str:String,_ i:Int) -> Character 46 { 47 return str[str.index(str.startIndex,offsetBy:i)] 48 } 49 }
