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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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Longest Arithmetic Subsequence — DP + Hash Map per Index
Find the longest arithmetic subsequence using dp[i][diff] = length ending at index i with given difference.
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Count Bad Pairs — Complement Counting with HashMap
Count pairs (i,j) where j-i != nums[j]-nums[i] by counting good pairs via frequency map of nums[i]-i.
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Make Sum Divisible by P — Prefix Mod + HashMap
Find the smallest subarray to remove so the remaining sum is divisible by p using prefix modular arithmetic.
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Tuple with Same Product — Product Frequency Map
Count tuples (a,b,c,d) where a*b=c*d by counting pairs with each product using a frequency map.
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