[Swift]LeetCode481. 神奇字符串 | Magical String

发布时间：2020-12-14 05:04:41 所属栏目：百科来源：网络整理

导读：A magical string?S?consists of only ‘1‘ and ‘2‘ and obeys the following rules: The string?S?is magical because concatenating the number of contiguous occurrences of characters ‘1‘ and ‘2‘ generates the string?Sitself. The first few

A magical string?S?consists of only ‘1‘ and ‘2‘ and obeys the following rules:

The string?S?is magical because concatenating the number of contiguous occurrences of characters ‘1‘ and ‘2‘ generates the string?Sitself.

The first few elements of string?S?is the following:?S?= "1221121221221121122……"

If we group the consecutive ‘1‘s and ‘2‘s in?S,it will be:

1 22 11 2 1 22 1 22 11 2 11 22 ......

and the occurrences of ‘1‘s or ‘2‘s in each group are:

1 2 2 1 1 2 1 2 2 1 2 2 ......

You can see that the occurrence sequence above is the?S?itself.

Given an integer N as input,return the number of ‘1‘s in the first N number in the magical string?S.

Note:?N will not exceed 100,000.

Example 1:

Input: 6
Output: 3
Explanation: The first 6 elements of magical string S is "12211" and it contains three 1‘s,so return 3.

神奇的字符串?S?只包含 ‘1‘ 和 ‘2‘，并遵守以下规则：

字符串?S?是神奇的，因为串联字符 ‘1‘ 和 ‘2‘ 的连续出现次数会生成字符串?S?本身。

字符串?S?的前几个元素如下：S?= “1221121221221121122 ......”

如果我们将?S?中连续的 1 和 2 进行分组，它将变成：

1 22 11 2 1 22 1 22 11 2 11 22 ......

并且每个组中 ‘1‘ 或 ‘2‘ 的出现次数分别是：

1 2 2 1 1 2 1 2 2 1 2 2 ......

你可以看到上面的出现次数就是?S?本身。

给定一个整数 N 作为输入，返回神奇字符串?S?中前 N 个数字中的 ‘1‘ 的数目。

注意：N 不会超过 100,000。

示例：

输入：6
输出：3
解释：神奇字符串 S 的前 6 个元素是 “12211”，它包含三个 1，因此返回 3。

Runtime:?20 ms

Memory Usage:?10.1 MB

 1 class Solution {
 2     func magicalString(_ n: Int) -> Int {
 3         if n <= 0 {return 0}
 4         if n <= 3 {return 1}
 5         var res:Int = 1
 6         var head:Int = 2
 7         var tail:Int = 3
 8         var num:Int = 1
 9         var v:[Int] = [1,2,2]
10         while(tail < n)
11         {
12             for i in 0..<v[head]
13             {
14                 v.append(num)
15                 if num == 1 && tail < n
16                 {
17                     res += 1
18                 }
19                 tail += 1
20             }
21             num ^= 3
22             head += 1
23         }
24         return res
25     }
26 }

124ms

 1 class Solution {
 2     func magicalString(_ n: Int) -> Int {
 3     var sequence = [Int]()
 4     sequence.append(contentsOf: [1,2])
 5     var groups = 2
 6     var result = 1
 7     if(n==0){return 0}
 8     while sequence.count<n {
 9         if(sequence[groups]==1){
10             result += sequence.last! == 1 ? 0:1
11             sequence.append(sequence.last! == 1 ? 2:1)
12             groups+=1
13         }else{
14             let temp = sequence.last! == 1 ? 2:1
15             sequence.append(contentsOf: [temp,temp])
16             result += temp == 1 ? 2:0
17             groups+=1
18         }
19     }
20     if(sequence.count==n){return result}else{
21         result -= sequence.last! == 1 ? 1:0
22         return result
23     }
24   }
25 }

（编辑：李大同）

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