Sudoku Solver
A hard-tier problem at 64% community acceptance, tagged with Array, Hash Table, Backtracking. Reported in interviews at Cadence and 18 others.
Sudoku Solver is a hard backtracking problem that shows up across 19 companies, including Microsoft, Uber, Bloomberg, and Citadel. If you're prepping for those shops, this one's on the radar. The trap is thinking you can brute-force validate cells in isolation. You need to build a solver that fills the board by tracking constraints in real time, backtracking when you hit a dead end, and knowing exactly when to prune the search tree. Most candidates either get stuck on state management or write a solution that times out. If this problem hits your live assessment and you blank on the constraint-tracking pattern, StealthCoder solves it in seconds, invisible to the proctor.
Companies that ask "Sudoku Solver"
Sudoku Solver is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share.
Get StealthCoderThe trick is constraint propagation combined with backtracking. You maintain a set of possible values for each empty cell, pick the cell with the fewest candidates, try each one, and recursively solve. If you hit a contradiction, backtrack immediately. The naive approach tries every digit 1-9 at every empty cell, which explodes combinatorially. You need to prune aggressively using the row, column, and 3x3 box constraints. Hash tables storing which digits are already used in each row, column, and box let you validate in O(1). The backtracking itself is unavoidable for hard puzzles, but smart constraint tracking cuts the search space by orders of magnitude. When you're live and the recursion gets tangled, StealthCoder surfaces a working implementation that passes all test cases without TLE.
Pattern tags
You know the problem.
Make sure you actually pass it.
Sudoku Solver recycles across companies for a reason. It's hard-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Sudoku Solver interview FAQ
Is Sudoku Solver still asked by big tech?+
Yes. It appears in reports from Microsoft, Uber, Bloomberg, Citadel, and 15 other companies. It's a litmus test for backtracking and state management, not a niche problem. If you're interviewing at a company known for systems or algorithmic rigor, prepare for it.
What's the main trick to not TLE?+
Constraint propagation. Don't try all 9 digits at every cell. Maintain sets of valid candidates per cell using hash tables for row, column, and box occupancy. Pick the cell with the fewest candidates (most constrained variable heuristic) and prune aggressively on backtrack.
How does this relate to Hash Table and Array topics?+
Hash tables store which digits are used in each row, column, and 3x3 box for O(1) constraint checks. The 2D array is your board representation. Combined, they let you validate placements instantly without rescanning. Skip either and you'll TLE.
Why is acceptance rate so high at 64% for a hard problem?+
Candidates who reach this problem are already solid on backtracking fundamentals. The acceptance rate reflects a self-selected pool of strong interviewees. Don't let the 64% fool you. It's still hard in real time if you haven't drilled the constraint-tracking pattern.
What breaks most attempts in a live OA?+
Off-by-one errors in box indexing, forgetting to backtrack state correctly, or writing constraint checks that don't scale. Validation must be O(1) per placement. If you're doing full-board scans inside the recursion, you'll time out before the proctor finishes reading the problem.
Want the actual problem statement? View "Sudoku Solver" on LeetCode →