dsa2 min read
First Unique Character in String — Two-Pass Frequency Map [Amazon Easy]
Find the index of the first non-repeating character in a string. Two-pass O(n) solution with a 26-array or HashMap.
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dsa3 min read
Valid Palindrome — Two Pointer Skip Non-Alphanumeric [Meta Easy]
Determine if a string is a palindrome considering only alphanumeric characters and ignoring cases. Two pointer O(n) O(1) solution with all 5 language implementations.
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dsa3 min read
Longest Common Prefix — Vertical Scan and Trie Approaches [Google Easy]
Find the longest common prefix string among an array of strings. Covers vertical scan O(n*m), horizontal scan, binary search, and Trie approaches with all 5 language solutions.
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dsa3 min read
Count Primes — Sieve of Eratosthenes O(n log log n) [Amazon Easy]
Count the number of prime numbers strictly less than n. Master the Sieve of Eratosthenes — one of the most elegant algorithms in CS — with full code in C, C++, Java, JavaScript and Python.
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dsa2 min read
Missing Number — XOR and Gauss Sum Two Solutions [Meta, Amazon Easy]
Find the missing number in [0,n]. Two elegant O(n) O(1) solutions: Gauss formula and XOR trick. Full solutions in C, C++, Java, JavaScript and Python.
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