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Find First Duplicate

CodeSignal - Interview Practice (Arrays)

Question Description. Link

Given an array a that contains only numbers in the range from 1 to a.length, find the first duplicate number for which the second occurrence has the minimal index. In other words, if there are more than 1 duplicated numbers, return the number for which the second occurrence has a smaller index than the second occurrence of the other number does. If there are no such elements, return -1.

Example

For a = [2, 1, 3, 5, 3, 2], the output should be firstDuplicate(a) = 3.
- There are 2 duplicates: numbers 2 and 3.
- The second occurrence of 3 has a smaller index than the second occurrence of 2 does, so the answer is 3.
For a = [2, 2], the output should be firstDuplicate(a) = 2.
For a = [2, 4, 3, 5, 1], the output should be firstDuplicate(a) = -1.

Solutions