61 Rotate List

class Solution:
    def decodeString(self, s: str) -> str:
        # time O(max(K)N) space O(max(K)N)
        # can't use tuple because tuple is immutable 
        st = [["",1]]
        num = ""
        for ch in s:
            #case 1: number
            if ch.isdigit():
                num += ch
            #case2: [, push "" num to stack, reset num
            elif ch == '[':
                st.append(["",num])
                num = ""
            # case3: ], pop and push partial complete string(ch * num) into stack  
            elif ch.isalpha():
                st[-1][0] += ch
            elif ch == ']':
                c,n = st.pop()
                st[-1][0] += c * int(n)
            # case4: alphabetic, keep update 
        return st[0][0]