Find the unique element in an array where all elements appear twice except one.
Examples:
[1, 1, 4, 4, 7, 8, 8, 29, 29] → 7[1, 2, 2, 3, 3] → 1
Summary (with approach)
Use XOR: since a ^ a = 0, a ^ 0 = a, and XOR is associative/commutative, XOR all numbers; the result is the single number. Runs in O(N) time and O(1) space. Alternative: hash counting in O(N) space.
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