442. Find All Duplicates in an Array

https://leetcode.com/problems/find-all-duplicates-in-an-array/

Problem

Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements that appear twice in this array.

Could you do it without extra space and in O(n) runtime?

Example:

Input:
[4,3,2,7,8,2,3,1]

Output:
[2,3]

Solution

class Solution {
public:
    vector<int> findDuplicates(vector<int>& nums) {
        vector<int> dup;
        dup.reserve(nums.size() / 2);
        for (int i = 0; i < nums.size(); ++i) {
            const int idx = abs(nums[i]) - 1;
            if (nums[idx] > 0) nums[idx] *= -1;
            else dup.emplace_back(idx + 1);
        }
        return dup;
    }
};