expert
14 min read
Regular Expression Matching
A greedy scan breaks at * because the pattern can take two legal futures: skip the quantified element, or consume one matching character and keep the same…
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Two-Dimensional Dynamic Programming
Model DP problems that naturally require two coordinates or progress dimensions.
Problems
Problems in Two-Dimensional Dynamic Programming
Practice problems that share this primary solution pattern and compare the clues that reveal it.
advanced
13 min read
Edit Distance
The table is easy to memorize and easy to misuse. The durable idea is simpler: track how far you have consumed each string, then let the final operation…
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advanced
12 min read
Interleaving String
When both source strings can provide the next target character, a greedy pointer has to guess. Dynamic programming keeps both possibilities alive until the…
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intermediate
11 min read
Minimum Path Sum
The right Minimum Path Sum solution is a two-dimensional dynamic program. For every coordinate, store the minimum sum needed to reach it from the top-left.…
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expert
13 min read
Scramble String
The difficult part of the Scramble String solution is not recognizing that characters are rearranged. It is preserving the recursive boundaries that made…
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advanced
12 min read
Unique Binary Search Trees II
The counting recurrence is only half the problem. To generate every tree, you must materialize every left/right subtree combination.
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intermediate
10 min read
Unique Paths
The reliable way to solve Unique Paths is to count paths to each cell, not to enumerate complete routes. Every cell has at most two meaningful…
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intermediate
12 min read
Unique Paths II
The recurrence is familiar. The interview usually turns on the cells you forgot: a blocked start, a blocked destination, or an obstacle that permanently…
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expert
13 min read
Wildcard Matching
The reliable way to solve wildcard matching is to model it as reachability over two consumed prefixes—not as an improvised backtracking fight with *.
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