Leetcode: Mini Parser

Posted on January 24, 2018September 10, 2019 by braindenny

Mini Parser

Similar Problems:

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.

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

## Blog link: https://code.dennyzhang.com/mini-parser
# """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
#class NestedInteger(object):
#    def __init__(self, value=None):
#        """
#        If value is not specified, initializes an empty list.
#        Otherwise initializes a single integer equal to value.
#        """
#
#    def isInteger(self):
#        """
#        @return True if this NestedInteger holds a single integer, rather than a nested list.
#        :rtype bool
#        """
#
#    def add(self, elem):
#        """
#        Set this NestedInteger to hold a nested list and adds a nested integer elem to it.
#        :rtype void
#        """
#
#    def setInteger(self, value):
#        """
#        Set this NestedInteger to hold a single integer equal to value.
#        :rtype void
#        """
#
#    def getInteger(self):
#        """
#        @return the single integer that this NestedInteger holds, if it holds a single integer
#        Return None if this NestedInteger holds a nested list
#        :rtype int
#        """
#
#    def getList(self):
#        """
#        @return the nested list that this NestedInteger holds, if it holds a nested list
#        Return None if this NestedInteger holds a single integer
#        :rtype List[NestedInteger]
#        """
class Solution(object):
    ## Basic Ideas: Stack
    ##     Whenever we found '[', DO NOT push. 
    ##         We initialize a empty NestedInteger as place holder
    ##     Whenever we found ']', we combine the last 2 place holers
    ##     Whenever we found digits/'-', look ahead to get what we need. 
    ##               Then construct NestedInteger, then combine it into last place holder
    ##
    ## Complexity: Time O(n), Space O(n)
    def deserialize(self, s):
        """
        :type s: str
        :rtype: NestedInteger
        """
        if len(s) == 0: return None
        if s.find('[') == -1: return NestedInteger(int(s))

        stack = []
        for word in s.split(','):
            i = 0
            while i< len(word):
                if word[i] == '[':
                    stack.append(NestedInteger())
                    i += 1
                elif word[i] == ']':
                    n1 = stack.pop()
                    n2 = stack.pop()
                    stack.append(n2+n1)
                    i += 1
                else:
                    # keep looking ahead until we get an non-digits
                    string = ''
                    while i<len(word) and (word[i].isdigit() or word[i] == '-'):
                        string = "%s%s" % (string, word[i])
                        i += 1
                    n = stack.pop()
                    stack.append(n + NestedInteger(int(string)))
        return stack[0]

    ## Basic Ideas: Stack
    ##              Whenever we found '[', push
    ##              Whenever we found ']', we keep poping until we find a '['
    ##
    ## Complexity: Time O(n), Space O(n)
    def deserialize(self, s):
        """
        :type s: str
        :rtype: NestedInteger
        """
        if len(s) == 0: return None
        if s.find('[') == -1: return NestedInteger(int(s))

        stack = []
        for word in s.split(','):
            num_str = ''
            for ch in word:
                if ch == '[':
                    stack.append(ch)
                    continue
                if ch != ']':
                    num_str = '%s%s' % (num_str, ch)
                else:
                    if num_str != '':
                        stack.append(NestedInteger(int(num_str)))
                        num_str = ''
                    # The sequence we get is right from left, but we need left from right.
                    l = []
                    while True:
                        element = stack.pop()
                        if element == '[':
                            break
                        l.insert(0, element)
                    n = NestedInteger() 
                    for element in l: n.add(element)
                    stack.append(n)
            if num_str != '':
                stack.append(NestedInteger(int(num_str)))
        return stack[0]

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