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Brick Wall [Medium] — Gap Position Frequency Map
Find the vertical line that crosses the fewest bricks by counting the most frequent gap position using a HashMap.
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Unique Number of Occurrences — Frequency Map Validation
Check if all value frequencies are unique using two hash sets in O(n).
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Count Wonderful Substrings — Bitmask + Prefix XOR Map
Count substrings with at most one odd-frequency letter using bitmask prefix XOR and a hash map.
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Equal Row and Column Pairs — Hash Tuple Rows and Columns
Count (row, col) pairs that are equal by hashing each row tuple and each column tuple.
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Two Sum IV - BST — Hash Set + In-Order Traversal
Find if a BST contains two nodes summing to target using a hash set during DFS traversal.
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