leetcode 385. Mini Parser

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Given a nested list of integers represented as a string, implement a parser to deserialize it.

Each element is either an integer, or a list – whose elements may also be integers or other lists.

Note:
You may assume that the string is well-formed:

  • String is non-empty.
  • String does not contain white spaces.
  • String contains only digits 0-9, [, - ,, ].

Example 1: 

Given s = "324",

You should return a NestedInteger object which contains a single integer 324.

Example 2:

Given s = "[123,[456,[789]]]",

Return a NestedInteger object containing a nested list with 2 elements:

1. An integer containing value 123.
2. A nested list containing two elements:
    i.  An integer containing value 456.
    ii. A nested list with one element:
         a. An integer containing value 789.

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class Solution(object):
    def deserialize(self, s):
        """
        :type s: str
        :rtype: NestedInteger
        """
        
        if s[0]!='[': return NestedInteger(int(s))
        
        stack=[NestedInteger()]
        num=''
        for i in s:
            if i=='[': 
                stack.append(NestedInteger())
            elif i==']':
                if num:
                    stack[-1].add(int(num))
                num=''
                buf=stack.pop()
                stack[-1].add(buf)
            elif i==',':
                if num:
                    stack[-1].add(int(num))
                num=''
            else:
                num+=i
        
        return stack[0].getList()[0]