Contains Duplicate
nums is a list of integers Return true when some number shows up more than once Return false when each number shows up only once Example: [4, 9, 4] => true (4 shows up twice) Example: [5, 6, 7] => false (no repeats)
Contains Duplicate is a classic scan-and-record problem: the right bookkeeping beats brute force. It's a great warm-up that builds data-structure instincts without a ton of moving parts. Watch your edge cases around repeats, and be clear about what your map is storing.
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