Find All Duplicates in an Array

Description

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

Idea: mark the existence of a number num by negating nums[num - 1].

class Solution {
public:
    vector<int> findDuplicates(vector<int>& nums) {
        vector<int> ans;
        for(int i = 0; i < nums.size(); ++i){
            int idx = abs(nums[i]) - 1;
            if(nums[idx] < 0)
                ans.push_back(idx + 1);
            else
                nums[idx] = -nums[idx];
        }
        // restore the array if the array cannot be changed when the function returns
        /*
        for(int &num : nums){
            if(num < 0)
                num = -num;
        }
        */
        return ans;
    }
};