481. Magical String

Description

Solution

Use a pointer to record current occurrence of next segment of s. The new added segment’s consecutive number should be different from back of s.

Code

class Solution:
    def magicalString(self, n: int) -> int:
        if n <= 3:
            return 1
        
        s = "122"
        freq,size,res = 2, 3, 1 #freq postion pointer
        while size < n:
            f = int(s[freq])
            c = 1 if s[len(s)-1] is "2" else 2
            s += str(c) * f
            if size + f >= n:
                if c == 1:
                    res += n - size
            else:
                if c == 1:
                    res += f
            size += f
            freq += 1
        return res