Find all Duplicates in an array (#442 Leetcode)

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 appears twice.

You must write an algorithm that runs in O(n) time and uses only constant extra space.

 

Example 1:

Input: nums = [4,3,2,7,8,2,3,1]
Output: [2,3]

Example 2:

Input: nums = [1,1,2]
Output: [1]

Example 3:

Input: nums = [1]
Output: []

 

Constraints:

• n == nums.length
• 1 <= n <= 105
• 1 <= nums[i] <= n
• Each element in nums appears once or twice.

From the problem statement, the constant extra space means the space for the return type.  

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