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 }
原文地址：https://www.cnblogs.com/strengthen/p/9936642.html
时间： 2024-10-29 13:17:34