今天快陣亡了，先放 code

class Solution:
    def decodeAtIndex(self, s: str, k: int) -> str:
        
        size = 0
        for c in s:
            if c.isdigit():
                size *= int(c)
            else:
                size += 1
                
        for c in s[::-1]:
            if c.isdigit():
                size //= int(c)
                k %= size
            else:
                if k == size or k == 0:
                    return c
                size -= 1