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Problem

Find All Duplicates

Easy
  • arrays
  • hash-table

Return every value that shows up more than once in arr. List each such value one time. The order of the result does not matter.

Example 1
Input
arr = [1, 2, 3, 2, 1]
Output
[1, 2]
Explanation

1 and 2 each appear twice. 3 appears once, so it is left out.

Example 2
Input
arr = [4, 4, 4, 5]
Output
[4]
Explanation

4 appears three times but is listed only once.

Example 3
Input
arr = [1, 2, 3]
Output
[]
Explanation

nothing repeats, so the result is empty.

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