Longest Word in Dictionary
Time O(n) · Space O(t) · Official statement on LeetCode
Solutions
// Time: O(n), n is the total sum of the lengths of words
// Space: O(t), t is the number of nodes in trie
class Solution {
public:
string longestWord(vector<string>& words) {
TrieNode trie;
for (int i = 0; i < words.size(); ++i) {
trie.Insert(words[i], i);
}
// DFS
stack<TrieNode *> stk;
for (const auto& node : trie.leaves) {
if (node) {
stk.emplace(node);
}
}
string result;
while (!stk.empty()) {
const auto curr = stk.top(); stk.pop();
if (curr->isString) {
const auto& word = words[curr->val];
if (word.size() > result.size() || (word.size() == result.size() && word < result)) {
result = word;
}
for (const auto& node : curr->leaves) {
if (node) {
stk.emplace(node);
}
}
}
}
return result;
}
private:
struct TrieNode {
bool isString;
int val;
vector<TrieNode *> leaves;
TrieNode() : isString{false}, val{0}, leaves(26) {}
void Insert(const string& s, const int i) {
auto* p = this;
for (const auto& c : s) {
if (!p->leaves[c - 'a']) {
p->leaves[c - 'a'] = new TrieNode;
}
p = p->leaves[c - 'a'];
}
p->isString = true;
p->val = i;
}
~TrieNode() {
for (auto& node : leaves) {
if (node) {
delete node;
}
}
}
};
};
Beginner Explanation
What is Longest Word in Dictionary?
Longest Word in Dictionary (LeetCode #720) is a Easy problem that primarily trains string.
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 trie.
- Only then translate the idea into code.
Why this problem matters
It builds core muscle memory you will reuse on harder variants. Official solution notes mention: Trie.
AlgoForge explanations are original teaching notes. Always open the official problem statement on LeetCode for constraints and examples.
Interview Walkthrough
Interview approach for Longest Word in Dictionary
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 trie.
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(n)) and space (O(t)) 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(n) time and O(t) space.
Pattern focus: trie
Use the pattern as a checklist:
- trie — confirm the invariant holds after each step
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(n) |
| Space | O(t) |
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 Longest Word in Dictionary
- Skipping edge cases — empty collections, single-element inputs, max constraints.
- Wrong invariant for trie — 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 trie:
- 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: string.
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 Longest Word in Dictionary in a second language (cpp, python).
- Drill 3–5 more problems tagged string.
- 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 trie 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
Longest Word in Dictionary (#720) — Easy. Pattern: trie. Complexity: O(n) time / O(t) space. Re-derive the invariant before coding.
FAQs
What is the time complexity of Longest Word in Dictionary?+
The reference solutions aim for O(n) time and O(t) space. Always re-derive complexity from the code you write in the interview.
What pattern does Longest Word in Dictionary use?+
It primarily maps to trie, within the broader topic of string.
Is Longest Word in Dictionary good for interviews?+
Yes — as a Easy 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/longest-word-in-dictionary/