n, where n is the length of the array tickets. Because of a glitch some tags were handed out twice (and some not at all). tickets lists every tag handed out; each number appears at most twice.
Return every number that appears exactly twice, in any order.
Try to do it in O(n) time and O(1) extra space, not counting the output (you may modify the array itself).tickets=[3, 1, 3, 4, 1, 6] → [3, 1]
Numbers 3 and 1 show up twice, all the others once.
tickets=[2, 2] → [2]
Number 2 was handed out twice and number 1 not at all.
tickets=[2, 3, 1] → []
All numbers are distinct, so there are no repeats.
n = len(tickets) ≤ 10⁵tickets[i] ≤ ntickets=[3, 1, 3, 4, 1, 6]
[3, 1]
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