Problem:
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 S itself.
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.
思路：
Solution (C++):
int magicalString(int n) {
string s = "122";
int i = 2;
while (s.size() < n) {
if (i & 1)
s.append(s[i++]-'0', '2');
else
s.append(s[i++]-'0', '1');
}
return count(s.begin(), s.begin()+n, '1');
}
性能：
Runtime: 24 ms Memory Usage: 7.6 MB
思路：
Solution (C++):
性能：
Runtime: ms Memory Usage: MB
