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: [1,2,2])
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 }