DSA TrainerFind All Duplicates in an Array
Given an integer array where each element is between 1 and n (inclusive), find all elements that appear twice. Solve in O(n) time and O(1) extra space (excluding output).
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Problem
Given an integer array nums of length n where all the integers of nums are in the range [1, n] and each integer appears once or twice, return an array of all the integers that appear twice.
Input
An integer array `nums` of length n where 1 <= nums[i] <= n.
Output
A list of all elements that appear exactly twice (any order).
Examples
Input: nums = [4,3,2,7,8,2,3,1]
Output: [2,3]
Input: nums = [1,1,2]
Output: [1]
Input: nums = [1]
Output: []
The brute-force approach
Count frequencies with a hash map. Return any element with count 2.
from collections import Counter
return [x for x, cnt in Counter(nums).items() if cnt == 2]O(n) time but O(n) extra space for the counter.
Spotting the pattern
This is a Cyclic Sort problem. The key question to ask yourself:
After cyclic sort, why is a duplicate sitting at the wrong index while the original copy is correctly placed?
Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.
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The Cyclic Sort walkthrough for Find All Duplicates in an Array: the progressive hint ladder, a row-by-row dry run, the optimized code, and an in-browser runner, plus both full courses, DSA and System Design, and every other premium problem.
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