Word Search
Time O(m * n * 3^l) · Space O(l) · Official statement on LeetCode
Solutions
// Time: O(m * n * 4 * 3^(l - 1)) ~= O(m * n * 3^l), l is the length of the word
// Space: O(l)
class Solution {
public:
bool exist(vector<vector<char> > &board, string word) {
const int m = board.size();
const int n = board.front().size();
vector<vector<bool> > visited(m, vector<bool> (n, false) );
for(int i = 0; i < m; ++i) {
for(int j = 0; j < n; ++j) {
if(dfs(board, word, 0, i, j, visited))
return true;
}
}
return false;
}
private:
bool dfs(vector<vector<char> > &board, string word, int index, int i, int j, vector<vector<bool> > &visited) {
if(index == word.size()) return true; // terminated condition
if(i < 0 || j < 0 || i >= board.size() || j >= board.front().size() // pruning
|| visited[i][j] || board[i][j] != word[index])
return false;
visited[i][j] = true; // set value
bool ret = dfs(board, word, index + 1, i + 1, j, visited)
|| dfs(board, word, index + 1, i, j + 1, visited)
|| dfs(board, word, index + 1, i - 1, j, visited)
|| dfs(board, word, index + 1, i, j - 1, visited);
visited[i][j] = false; // recover value
return ret;
}
};
Beginner Explanation
What is Word Search?
Word Search (LeetCode #79) is a Medium problem that primarily trains backtracking.
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 dfs backtracking.
- 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 Word Search
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 dfs backtracking.
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(m * n * 3^l)) and space (O(l)) 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(m * n * 3^l) time and O(l) space.
Pattern focus: dfs backtracking
Use the pattern as a checklist:
- dfs backtracking — 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(m * n * 3^l) |
| Space | O(l) |
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 Word Search
- Skipping edge cases — empty collections, single-element inputs, max constraints.
- Wrong invariant for dfs backtracking — 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 dfs backtracking:
- 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: backtracking.
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 Word Search in a second language (cpp, python).
- Drill 3–5 more problems tagged backtracking.
- 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 dfs backtracking 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
Word Search (#79) — Medium. Pattern: dfs backtracking. Complexity: O(m * n * 3^l) time / O(l) space. Re-derive the invariant before coding.
FAQs
What is the time complexity of Word Search?+
The reference solutions aim for O(m * n * 3^l) time and O(l) space. Always re-derive complexity from the code you write in the interview.
What pattern does Word Search use?+
It primarily maps to dfs backtracking, within the broader topic of backtracking.
Is Word Search 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/word-search/