intermediate
10 min read
Generate Parentheses
Generate only prefixes that can still become valid. The balance state tells you exactly which branches to keep.
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Constraint Tracking
Maintain compact state representing multiple simultaneous rules that candidate values, placements, or paths must satisfy.
Tagged articles
Constraint Tracking articles
8 articles in this tag.
advanced
13 min read
Minimum Window Substring
The hard part is not moving two pointers. It is preserving the target’s multiplicity while the window changes.
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advanced
13 min read
N-Queens II
The key change from N-Queens is the output contract: you need one integer, so the search should retain only reversible constraints and count valid leaves.
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advanced
13 min read
N-Queens
The board is only the output surface. The real N-Queens solution is a depth-n search over column assignments, with three constraints checked before each…
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intermediate
11 min read
Permutations
The search tree is easy to recognize and easy to corrupt. Build one position at a time, choose an unused value, recurse, then undo exactly that choice.
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expert
14 min read
Sudoku Solver
The phrase “try digits and backtrack” is the easy part. The interview-grade solution keeps four representations synchronized: the board, row constraints,…
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intermediate
8 min read
Valid Sudoku
A Sudoku validator does not solve the puzzle. It tracks whether the digits already placed violate any row, column, or 3×3 box constraint.
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intermediate
14 min read
Word Search
A grid DFS can match the right letters and still be wrong. The missing piece is path-local state: mark a cell when you enter it, explore from that choice,…
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