Interview AiBox logo
Interview AiBox
#385
Medium
auto_awesomeStack-based state management

LeetCode Problem Workspace

Mini Parser

Deserialize a nested list string using stack-based state management, handling integers and nested lists with depth-first parsing.

stringstackdepth first search
category

3

Topics

code_blocks

5

Code langs

hub

3

Related

play_arrowStart Solutionarrow_forward
speedComplexityopen_in_newLeetCode

Practice Focus

Medium · Stack-based state management

bolt

Answer-first summary

Deserialize a nested list string using stack-based state management, handling integers and nested lists with depth-first parsing.

Interview AiBox logo

Ace coding interviews with Interview AiBox guidance for Stack-based state management

Try AiBox Copilotarrow_forward

Start by iterating through the input string while maintaining a stack to track nested structures. Push new NestedInteger objects when encountering brackets and finalize them when closing brackets appear. Direct integers are parsed immediately, ensuring correct placement in the current list context, giving a robust solution for Mini Parser's stack-based state challenges.

Problem Statement

You are given a string s representing a serialized nested list of integers. Implement a parser that converts this string into a NestedInteger object. Each element in the string is either a single integer or another list, which can recursively contain integers or nested lists.

The input string may include digits, negative signs, commas, and square brackets. Your parser should correctly handle multi-level nesting and return the deserialized NestedInteger structure. For example, s = "324" should return a single integer, while s = "[123,[456,[789]]]" should return a nested list structure reflecting the hierarchy of integers.

Examples

Example 1

Input: s = "324"

Output: 324

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

Example 2

Input: s = "[123,[456,[789]]]"

Output: [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

Constraints

  • 1 <= s.length <= 5 * 104
  • s consists of digits, square brackets "[]", negative sign '-', and commas ','.
  • s is the serialization of valid NestedInteger.
  • All the values in the input are in the range [-106, 106].

Solution Approach

Stack-Based Iterative Parsing

Use a stack to manage the current NestedInteger at each level. When encountering '[', push a new NestedInteger onto the stack. On ']', pop the top NestedInteger and append it to the previous one. Integers are parsed and added directly to the current top of the stack.

Handle Integers and Negative Signs

Track the start of a number and handle optional negative signs. When a comma or closing bracket is reached, convert the substring to an integer and add it to the current NestedInteger. This ensures single integers are correctly captured and nested at the appropriate level.

Depth-First Placement

Ensure that when a nested list is completed, it is appended to the enclosing list. This depth-first strategy guarantees that deeply nested structures maintain their hierarchy and prevents misplacement of integers in sibling lists.

Complexity Analysis

Metric Value
Time Depends on the final approach
Space Depends on the final approach

Time complexity is O(n) where n is the length of the string, as each character is processed once. Space complexity is O(d) for the stack, where d is the maximum depth of nesting, to track active NestedInteger objects.

What Interviewers Usually Probe

  • Expect candidates to manage multiple nesting levels without losing context.
  • Watch if they correctly handle negative integers and multi-digit numbers.
  • Check for proper stack use to maintain current parsing state.

Common Pitfalls or Variants

Common pitfalls

  • Failing to handle single integers outside brackets correctly.
  • Incorrectly appending integers to the wrong NestedInteger after closing a bracket.
  • Ignoring negative signs or multi-digit numbers when parsing integers.

Follow-up variants

  • Parsing strings with additional whitespace or non-standard separators.
  • Supporting optional null or empty lists within the nested structure.
  • Handling extremely deep nesting requiring recursion or explicit stack management.

FAQ

What is the main challenge in Mini Parser?

The core challenge is correctly managing stack-based state to parse integers and nested lists while maintaining hierarchy.

Can Mini Parser handle negative integers?

Yes, the parser must detect negative signs and include them when constructing integer values.

How does the stack help in parsing nested lists?

The stack keeps track of the current NestedInteger context at each level, ensuring proper placement of nested elements.

What is the expected output for s = "[123,[456,[789]]]"?

A NestedInteger object containing two elements: integer 123 and a nested list [456,[789]] preserving the hierarchy.

Why is Mini Parser considered a stack-based state management problem?

Because parsing requires tracking multiple active nested lists and their elements simultaneously, which is naturally handled by a stack.

terminal

Solution

Solution 1: Recursion

We first judge whether the string $s$ is empty or an empty list. If so, simply return an empty `NestedInteger`. If $s$ is an integer, we simply return a `NestedInteger` containing this integer. Otherwise, we traverse the string $s$ from left to right. If the current depth is $0$ and we encounter a comma or the end of the string $s$, we take a substring and recursively call the function to parse the substring and add the return value to the list. Otherwise, if the current encounter is a left parenthesis, we increase the depth by $1$ and continue to traverse. If we encounter a right parenthesis, we decrease the depth by $1$ and continue to traverse.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
# """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
# class NestedInteger:
#    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:
    def deserialize(self, s: str) -> NestedInteger:
        if not s or s == '[]':
            return NestedInteger()
        if s[0] != '[':
            return NestedInteger(int(s))
        ans = NestedInteger()
        depth, j = 0, 1
        for i in range(1, len(s)):
            if depth == 0 and (s[i] == ',' or i == len(s) - 1):
                ans.add(self.deserialize(s[j:i]))
                j = i + 1
            elif s[i] == '[':
                depth += 1
            elif s[i] == ']':
                depth -= 1
        return ans

Solution 2: Stack

We can use a stack to simulate the recursive process.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
# """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
# class NestedInteger:
#    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:
    def deserialize(self, s: str) -> NestedInteger:
        if not s or s == '[]':
            return NestedInteger()
        if s[0] != '[':
            return NestedInteger(int(s))
        ans = NestedInteger()
        depth, j = 0, 1
        for i in range(1, len(s)):
            if depth == 0 and (s[i] == ',' or i == len(s) - 1):
                ans.add(self.deserialize(s[j:i]))
                j = i + 1
            elif s[i] == '[':
                depth += 1
            elif s[i] == ']':
                depth -= 1
        return ans

Continue Practicing

#388 longest absolute file path#399 evaluate division#402 remove k digits
category

Continue Topic

string

Practice more edge cases under the same topic.

arrow_forwardauto_awesome

Continue Pattern

Stack-based state management

Expand the same solving frame across more problems.

arrow_forwardsignal_cellular_alt

Same Difficulty Track

Medium

Stay on this level to stabilize interview delivery.

arrow_forward
Mini Parser Solution: Stack-based state management | LeetCode #385 Medium