Find the unique element in an array where all other elements appear twice except one.
Examples:
[1, 1, 4, 4, 7, 8, 8, 29, 29] -> 7
[1, 2, 2, 3, 3] -> 1The key idea is to identify the one number that appears only once while every other number appears exactly twice. A straightforward solution uses a hash map to count frequencies, then returns the value with count 1. A more optimal solution uses XOR: equal numbers cancel each other out, and XOR with 0 preserves the value, so XOR-ing all elements leaves the unique element. This is a classic interview problem that checks understanding of bitwise operations and O(n) time, O(1) extra space solutions.
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