★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
➤微信公众号：山青咏芝（shanqingyongzhi）
➤博客园地址：山青咏芝（https://www.cnblogs.com/strengthen/）
➤GitHub地址：https://github.com/strengthen/LeetCode
➤原文地址：https://www.cnblogs.com/strengthen/p/9936642.html
➤如果链接不是山青咏芝的博客园地址，则可能是爬取作者的文章。
➤原文已修改更新！强烈建议点击原文地址阅读！支持作者！支持原创！
★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
A message containing letters from A-Z is being encoded to numbers using the following mapping:
'A' -> 1 'B' -> 2 ... 'Z' -> 26
Given a non-empty string containing only digits, determine the total number of ways to decode it.
Example 1:
Input: "12" Output: 2 Explanation: It could be decoded as "AB" (1 2) or "L" (12).
Example 2:
Input: "226" Output: 3 Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
一条包含字母 A-Z 的消息通过以下方式进行了编码：
'A' -> 1 'B' -> 2 ... 'Z' -> 26
给定一个只包含数字的非空字符串，请计算解码方法的总数。
示例 1:
输入: "12" 输出: 2 解释: 它可以解码为 "AB"（1 2）或者 "L"（12）。
示例 2:
输入: "226" 输出: 3 解释: 它可以解码为 "BZ" (2 26), "VF" (22 6), 或者 "BBF" (2 2 6) 。
8ms
1 class Solution { 2 func numDecodings(_ s: String) -> Int { 3 if s.isEmpty || s[s.startIndex] == "0" { return 0 } 4 if s.count == 1 { return 1 } 5 var count = 1, last = 1 6 var i = s.index(after: s.startIndex) 7 while i < s.endIndex { 8 var cur = 0 9 if s[i] > "0" { 10 cur += count 11 } 12 if s[s.index(before: i)] == "1" || (s[s.index(before: i)] == "2" && s[i] <= "6") { 13 cur += last 14 } 15 last = count 16 count = cur 17 if last == 0 { 18 return 0 19 } 20 i = s.index(after: i) 21 } 22 return count 23 } 24 }
16ms
1 class Solution { 2 func numDecodings(_ s: String) -> Int { 3 guard s.count > 0 else { 4 return 0 5 } 6 7 let digits = Array(s).map { Int(String($0))! } 8 var dp = Array(repeating: 0, count: digits.count + 1) 9 dp[0] = 1 10 dp[1] = digits[0] == 0 ? 0 : 1 11 for i in stride(from: 2, through: digits.count, by: 1) { 12 dp[i] += (digits[i - 1] > 0 ? dp[i - 1] : 0) 13 let num = digits[i - 2] * 10 + digits[i - 1] 14 dp[i] += ((num >= 10 && num <= 26) ? dp[i - 2] : 0) 15 } 16 17 return dp[digits.count] 18 } 19 }
20ms
1 class Solution { 2 func numDecodings(_ s: String) -> Int { 3 guard !s.isEmpty && s.first! != "0" else { 4 return 0 5 } 6 7 if s.count == 1 { 8 return 1 9 } 10 11 var sum: [Int] = [1] 12 13 let first = s[s.startIndex] 14 let second = s[s.index(after: s.startIndex)] 15 if second == "0" && first >= "3" { 16 return 0 17 } 18 19 let str = String(first) + String(second) 20 if str == "10" || str == "20" || str > "26" { 21 sum.append(1) 22 } 23 else { 24 sum.append(2) 25 } 26 27 for i in 2..<s.count { 28 let index = String.Index(encodedOffset: i) 29 let lastIndex = String.Index(encodedOffset: i-1) 30 if s[index] == "0" && (s[lastIndex] == "0" || s[lastIndex] >= "3") { 31 return 0 32 } 33 34 if s[index] == "0" { 35 sum.append(sum[i-2]) 36 } 37 else { 38 if s[lastIndex] == "1" || (s[lastIndex] == "2" && s[index] <= "6") { 39 sum.append(sum[i-1] + sum[i-2]) 40 } 41 else { 42 sum.append(sum[i-1]) 43 } 44 } 45 } 46 47 return sum.last! 48 } 49 }
28ms
1 class Solution { 2 func numDecodings(_ s: String) -> Int { 3 if s.count == 0 { 4 return 0 5 } 6 7 let n = s.count 8 var s = s 9 var cache = [Int](repeating: -1, count: n + 1) 10 cache[n] = 1 11 return num(0, &s, &cache) 12 } 13 14 fileprivate func num(_ i: Int, _ s: inout String, _ cache: inout [Int]) -> Int { 15 if cache[i] > -1 { 16 return cache[i] 17 } 18 if s[i] == "0" { 19 cache[i] = 0 20 return 0 21 } 22 var res = num(i + 1, &s, &cache) 23 if i < s.count - 1 && (s[i] == "1" || s[i] == "2" && s[i + 1] < "7") { 24 res += num(i + 2, &s, &cache) 25 } 26 cache[i] = res 27 28 return res 29 } 30 } 31 32 extension String { 33 subscript (i: Int) -> Character { 34 return self[index(startIndex, offsetBy: i)] 35 } 36 }
32ms
1 class Solution { 2 3 func numDecodings(_ s: String) -> Int { 4 5 guard s.count > 0 else { 6 7 return 0 8 } 9 10 guard s.count > 1 else { 11 12 if s[0] == "0" { 13 14 return 0 15 } 16 17 return 1 18 } 19 var dp = Array(repeating: 0, count: s.count + 1) 20 dp[s.count] = 1 21 dp[s.count - 1] = s[s.count - 1] == "0" ? 0 : 1 22 23 for i in (0...s.count - 2).reversed() { 24 25 if s[i] == "0" { 26 27 continue 28 } 29 30 dp[i] = Int(s[i..<i+2])! <= 26 ? dp[i + 1] + dp[i + 2] : dp[i + 1] 31 } 32 33 return dp[0] 34 } 35 } 36 37 extension String { 38 39 subscript (i: Int) -> Character { 40 41 return self[index(startIndex, offsetBy: i)] 42 } 43 44 subscript (bounds: CountableRange<Int>) -> String { 45 46 let start = index(startIndex, offsetBy: bounds.lowerBound) 47 let end = index(startIndex, offsetBy: bounds.upperBound) 48 return String(self[start ..< end]) 49 } 50 }
36ms
1 class Solution { 2 func numDecodings(_ s: String) -> Int { 3 guard s.count > 0 else { 4 return 0 5 } 6 var result = [Int](repeating: 1, count: s.count + 1) 7 result[s.count - 1] = (Int(String(s[s.count - 1])) ?? 0) > 0 ? 1 : 0 8 for index in stride(from: s.count - 2, through: 0, by: -1) { 9 if ((Int(String(s[index])) ?? 0) == 0) { 10 result[index] = 0 11 } else if ((Int(String(s[index...index + 1])) ?? 0) > 26) { 12 result[index] = result[index + 1] 13 } else { 14 result[index] = result[index + 1] + result[index + 2] 15 } 16 } 17 return result[0] 18 } 19 } 20 21 extension String { 22 subscript (i: Int) -> Character { 23 return self[index(startIndex, offsetBy: i)] 24 } 25 subscript (bounds: CountableRange<Int>) -> Substring { 26 let start = index(startIndex, offsetBy: bounds.lowerBound) 27 let end = index(startIndex, offsetBy: bounds.upperBound) 28 return self[start ..< end] 29 } 30 subscript (bounds: CountableClosedRange<Int>) -> Substring { 31 let start = index(startIndex, offsetBy: bounds.lowerBound) 32 let end = index(startIndex, offsetBy: bounds.upperBound) 33 return self[start ... end] 34 } 35 subscript (bounds: CountablePartialRangeFrom<Int>) -> Substring { 36 let start = index(startIndex, offsetBy: bounds.lowerBound) 37 let end = index(endIndex, offsetBy: -1) 38 return self[start ... end] 39 } 40 subscript (bounds: PartialRangeThrough<Int>) -> Substring { 41 let end = index(startIndex, offsetBy: bounds.upperBound) 42 return self[startIndex ... end] 43 } 44 subscript (bounds: PartialRangeUpTo<Int>) -> Substring { 45 let end = index(startIndex, offsetBy: bounds.upperBound) 46 return self[startIndex ..< end] 47 } 48 } 49 50 extension Substring { 51 subscript (i: Int) -> Character { 52 return self[index(startIndex, offsetBy: i)] 53 } 54 subscript (bounds: CountableRange<Int>) -> Substring { 55 let start = index(startIndex, offsetBy: bounds.lowerBound) 56 let end = index(startIndex, offsetBy: bounds.upperBound) 57 return self[start ..< end] 58 } 59 subscript (bounds: CountableClosedRange<Int>) -> Substring { 60 let start = index(startIndex, offsetBy: bounds.lowerBound) 61 let end = index(startIndex, offsetBy: bounds.upperBound) 62 return self[start ... end] 63 } 64 subscript (bounds: CountablePartialRangeFrom<Int>) -> Substring { 65 let start = index(startIndex, offsetBy: bounds.lowerBound) 66 let end = index(endIndex, offsetBy: -1) 67 return self[start ... end] 68 } 69 subscript (bounds: PartialRangeThrough<Int>) -> Substring { 70 let end = index(startIndex, offsetBy: bounds.upperBound) 71 return self[startIndex ... end] 72 } 73 subscript (bounds: PartialRangeUpTo<Int>) -> Substring { 74 let end = index(startIndex, offsetBy: bounds.upperBound) 75 return self[startIndex ..< end] 76 } 77 }
48ms
1 class Solution { 2 3 func decodeHelperDynamic(str: String, n: Int, memo: inout [Int]) -> Int { 4 if n == 0 { return 1 } 5 6 let s = str.count - n 7 let sIndex = str.index(str.startIndex, offsetBy: s) 8 if str[sIndex] == "0" { return 0 } 9 10 if memo[n] != -1 { 11 return memo[n] 12 } 13 14 var result = decodeHelperDynamic(str: str, n: n - 1, memo: &memo) 15 if n >= 2 { 16 let startIndex = str.index(str.startIndex, offsetBy: s) 17 let finishIndex = str.index(str.startIndex, offsetBy: s + 2) 18 let subStr = str[startIndex..<finishIndex] 19 if let subInt = Int(subStr), subInt <= 26 { 20 result += decodeHelperDynamic(str: str, n: n - 2, memo: &memo) 21 } 22 } 23 memo[n] = result 24 return result 25 } 26 27 func numDecodings(_ s: String) -> Int { 28 29 let n = s.count 30 var memo = Array(repeating: -1, count: n + 1) 31 return decodeHelperDynamic(str: s, n: n, memo: &memo) 32 33 } 34 }