[Swift]LeetCode842. 将数组拆分成斐波那契序列 | Split Array into Fibonacci Sequence

Given a string S of digits, such as S = "123456579", we can split it into a Fibonacci-like sequence [123, 456, 579].

Formally, a Fibonacci-like sequence is a list F of non-negative integers such that:

• 0 <= F[i] <= 2^31 - 1, (that is, each integer fits a 32-bit signed integer type);
• F.length >= 3;
• and F[i] + F[i+1] = F[i+2] for all 0 <= i < F.length - 2.

Also, note that when splitting the string into pieces, each piece must not have extra leading zeroes, except if the piece is the number 0 itself.

Return any Fibonacci-like sequence split from S, or return [] if it cannot be done.

Example 1:

Input: "123456579"
Output: [123,456,579]

Example 2:

Input: "11235813"
Output: [1,1,2,3,5,8,13]

Example 3:

Input: "112358130"
Output: []
Explanation: The task is impossible.

Example 4:

Input: "0123"
Output: []
Explanation: Leading zeroes are not allowed, so "01", "2", "3" is not valid.

Example 5:

Input: "1101111"
Output: [110, 1, 111]
Explanation: The output [11, 0, 11, 11] would also be accepted.

Note:

1. 1 <= S.length <= 200
2. S contains only digits.

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给定一个数字字符串 S，比如 S = "123456579"，我们可以将它分成斐波那契式的序列 [123, 456, 579]。

形式上，斐波那契式序列是一个非负整数列表 F，且满足：

• 0 <= F[i] <= 2^31 - 1，（也就是说，每个整数都符合 32 位有符号整数类型）；
• F.length >= 3；
• 对于所有的0 <= i < F.length - 2，都有 F[i] + F[i+1] = F[i+2]成立。

另外，请注意，将字符串拆分成小块时，每个块的数字一定不要以零开头，除非这个块是数字 0 本身。

返回从 S 拆分出来的所有斐波那契式的序列块，如果不能拆分则返回 []。

示例 1：

输入："123456579"
输出：[123,456,579]

示例 2：

输入: "11235813"
输出: [1,1,2,3,5,8,13]

示例 3：

输入: "112358130"
输出: []
解释: 这项任务无法完成。

示例 4：

输入："0123"
输出：[]
解释：每个块的数字不能以零开头，因此 "01"，"2"，"3" 不是有效答案。

示例 5：

输入: "1101111"
输出: [110, 1, 111]
解释: 输出 [11,0,11,11] 也同样被接受。

提示：

1. 1 <= S.length <= 200
2. 字符串 S 中只含有数字。

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待完善

 1 class Solution {
 2     func splitIntoFibonacci(_ S: String) -> [Int] {         
 3         var res:[Int] = [Int]()
 4         backtracking(S, 0, &res)
 5         return res
 6     }
 7     
 8     func backtracking(_ S:String,_ start:Int,_ res:inout [Int]) -> Bool
 9     {
10         var n:Int = S.count
11         if start >= n && res.count >= 3
12         {
13             return true
14         }
15         var maxSplitSize:Int = (S[start] == "0") ? 1 : 10
16         for i in 1...maxSplitSize
17         {
18             if start + i <= S.count
19             {
20                 let num:Int = Int(S.subString(start, i)) ?? 0
21                 if num > Int(Int16.max)
22                 {
23                     continue
24                 }
25                 var sz:Int = res.count
26                 if sz >= 2 && res[sz - 1] + res[sz - 2] != num
27                 {
28                     continue
29                 }
30                 res.append(num)
31                 if backtracking(S, start + i, &res)
32                 {
33                     return true
34                 }
35                 res.popLast() 
36             }
37             
38         }
39         return false
40     }
41 }
42 
43 extension String {
44     //subscript函数可以检索数组中的值
45     //直接按照索引方式截取指定索引的字符
46     subscript (_ i: Int) -> Character {
47         //读取字符
48         get {return self[index(startIndex, offsetBy: i)]}
49     }
50     
51     // 截取字符串：指定索引和字符数
52     // - begin: 开始截取处索引
53     // - count: 截取的字符数量
54     func subString(_ begin:Int,_ count:Int) -> String {
55         let start = self.index(self.startIndex, offsetBy: max(0, begin))
56         let end = self.index(self.startIndex, offsetBy:  min(self.count, begin + count))
57         return String(self[start..<end]) 
58     }
59 }