442.cpp

class Solution {
public:
    vector<int> findDuplicates(vector<int>& nums) {
        vector<int> res;
        for(int i = 0; i < nums.size(); i++) {
            nums[abs(nums[i]) - 1] = -nums[abs(nums[i]) - 1];
            if (nums[abs(nums[i]) - 1] > 0) res.push_back(abs(nums[i]));
        }
        return res;
    }
};