394. Decode String

class Solution:
    def decodeString(self, s: str) -> str:
        st=[]
        for i in s:
            #if not '[' just add
            if i!=']':
                st.append(i)
            else:
                # Sinec we got ] we have to pop till we make a block enclosing [ and store it in subs
                subs=''
                while st[-1]!='[':
                    subs=st.pop()+subs
                st.pop()
                #now check any number preceding this block. Note this number might have more than one digit. So check
                #till its preceding character is no more a digit
                #Then multiply this number with the block and add to stack again.
                num=''
                while st and st[-1].isdigit():
                    num=st.pop()+num
                st.append(subs*int(num))
        #Now currently stack holds all independent blocks and just return them by joining
        return ''.join(st)