481. Magical String

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.

# @lc code=start
using LeetCode

function magical_string(n::Int)
    res = "122"
    idx = 2
    while length(res) < n
        res *= '1'^(res[idx+=1] - '0')
        res *= '2'^(res[idx += 1] - '0')
    end
    return count(==('1'), SubString(res, 1, n))
end
# @lc code=end

magical_string (generic function with 1 method)

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