Design Phone Directory
Time O(1) · Space O(n) · Official statement on LeetCode
Solutions
// init: Time: O(n), Space: O(n)
// get: Time: O(1), Space: O(1)
// check: Time: O(1), Space: O(1)
// release: Time: O(1), Space: O(1)
class PhoneDirectory {
public:
/** Initialize your data structure here
@param maxNumbers - The maximum numbers that can be stored in the phone directory. */
PhoneDirectory(int maxNumbers) :
curr_{0}, numbers_(maxNumbers), used_(maxNumbers) { // Time: O(n), Space: O(n)
iota(numbers_.begin(), numbers_.end(), 0);
}
/** Provide a number which is not assigned to anyone.
@return - Return an available number. Return -1 if none is available. */
int get() { // Time: O(1), Space: O(1)
if (curr_ == numbers_.size()) {
return -1;
}
const auto number = numbers_[curr_++];
used_[number] = true;
return number;
}
/** Check if a number is available or not. */
bool check(int number) { // Time: O(1), Space: O(1)
if (number < 0 || number >= numbers_.size()) {
return false;
}
return !used_[number];
}
/** Recycle or release a number. */
void release(int number) { // Time: O(1), Space: O(1)
if (number < 0 || number >= numbers_.size() || !used_[number]) {
return;
}
used_[number] = false;
numbers_[--curr_] = number ;
}
private:
int curr_;
vector<int> numbers_;
vector<bool> used_;
};
/**
* Your PhoneDirectory object will be instantiated and called as such:
* PhoneDirectory obj = new PhoneDirectory(maxNumbers);
* int param_1 = obj.get();
* bool param_2 = obj.check(number);
* obj.release(number);
*/
Beginner Explanation
What is Design Phone Directory?
Design Phone Directory (LeetCode #379) is a Medium problem that primarily trains design.
How to think about it
- Restate the goal in your own words before coding.
- Work a tiny example by hand so the invariant becomes obvious.
- Identify the pattern — this problem aligns with general problem-solving.
- Only then translate the idea into code.
Why this problem matters
It sits in the sweet spot of interview difficulty: multiple valid approaches, clear trade-offs.
AlgoForge explanations are original teaching notes. Always open the official problem statement on LeetCode for constraints and examples.
Interview Walkthrough
Interview approach for Design Phone Directory
Opening (30–60 seconds)
- Clarify inputs/outputs and edge cases (empty input, single element, duplicates, overflow).
- State a brute force so the interviewer knows you can solve it naively.
- Propose the optimal direction tied to general problem-solving.
Core solution narrative
- Define the state you track (pointers, DP cell, set membership, stack top, etc.).
- Explain the transition when you process the next element.
- Call out time (O(1)) and space (O(n)) before coding.
- Code cleanly; narrate variable names.
What interviewers listen for
- Correctness on edge cases
- Complexity honesty
- Ability to discuss trade-offs (e.g., hash map space vs. sort + two pointers)
Follow-up questions they may ask
- Can you solve it with less memory?
- What if the input stream is infinite / doesn't fit in RAM?
- How would tests look for adversarial inputs?
Optimized Approach
Optimized solution notes
The reference solutions on AlgoForge target O(1) time and O(n) space.
Pattern focus: general problem-solving
Use the pattern as a checklist:
- Identify the dominant pattern and stick to one clear invariant
Multiple methods appear in the source solutions — compare them and explain when each is preferable.
Implementation tips
- Prefer readable names over micro-optimizations in interviews.
- Extract helpers only when they clarify (e.g., expand-around-center, DFS visit).
- After AC-level logic, re-scan for off-by-one and null checks.
Complexity Analysis
Complexity
| Measure | Bound |
|---|---|
| Time | O(1) |
| Space | O(n) |
How to justify this in an interview
- Time: count loops, map/set operations, and recursive branching; state average vs worst case if relevant.
- Space: include hash maps, recursion stack, and output allocation when the problem asks for it.
If your implementation differs from the reference, re-derive big-O from your code — never memorize a complexity you cannot defend.
Common Mistakes
Common mistakes on Design Phone Directory
- Skipping edge cases — empty collections, single-element inputs, max constraints.
- Wrong invariant for general problem-solving — updating state too early or too late.
- Mutating input unexpectedly when the problem forbids it.
- Off-by-one in windows, ranges, or binary search bounds.
- Ignoring overflow / precision for integer arithmetic problems.
- Overengineering — jumping to an advanced structure when a simpler approach works.
Alternative Approaches
Alternatives
The source file includes more than one method. Compare:
- Primary optimized path — best complexity for typical interviews.
- Secondary approach — often brute force, sorting-based, or space-optimized variant.
Practice articulating when you would pick each (constraints, readability, follow-ups).
Edge Cases
Edge cases checklist
- Minimum input size
- Maximum input size / time limits
- Duplicates and already-sorted input
- Negative numbers / zeros (if applicable)
- Disconnected structures (graphs/trees)
- Single path vs branching recursion depth
Pattern Recognition
Spotting this pattern
Signal phrases that point to general problem-solving:
- Sorted input or ability to sort without changing the answer class
- Need for contiguous subarray / substring → consider sliding window
- Need for O(1) membership → hash set/map
- Optimal substructure + overlapping subproblems → DP
- Connectivity / components → graph DFS/BFS or Union-Find
Primary topics: design.
Follow-up Interview Questions
Follow-ups
- How does the solution change if the input is a stream?
- Can you solve it in-place?
- What if duplicates must be handled differently?
- How would you parallelize the approach?
- Design tests that would break a buggy implementation.
Practice Recommendations
What to practice next
- Re-solve Design Phone Directory in a second language (cpp, python).
- Drill 3–5 more problems tagged design.
- Teach the solution out loud in under 5 minutes.
- Add this problem to your revision calendar in 3 days and 14 days.
Visualization
Study checklist
- Read the official problem statement on LeetCode
- Solve on paper / whiteboard first
- Implement the general problem-solving approach
- Verify edge cases from the checklist
- State time and space complexity aloud
- Compare with the AlgoForge reference solution
- Schedule a revision session
Revision notes
Design Phone Directory (#379) — Medium. Pattern: general problem-solving. Complexity: O(1) time / O(n) space. Re-derive the invariant before coding.
FAQs
What is the time complexity of Design Phone Directory?+
The reference solutions aim for O(1) time and O(n) space. Always re-derive complexity from the code you write in the interview.
What pattern does Design Phone Directory use?+
It primarily maps to general problem-solving, within the broader topic of design.
Is Design Phone Directory good for interviews?+
Yes — as a Medium problem it is a solid practice target. Pair it with related problems in the same pattern family for spaced repetition.
Where can I read the official statement?+
Open the official LeetCode page for constraints and examples: https://leetcode.com/problems/design-phone-directory/