Approach-3) Find Duplicates - Negative Marking

find_duplicates_app_3.cpp

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        for(int i=0; i<nums.size(); i++){
            int absNum = abs(nums[i]);
            if(nums[absNum] < 0){
                // Number is duplicate
                return absNum;
            }
            nums[absNum] = -nums[absNum];
        }

        for(int i=0; i<nums.size(); i++){
            if(nums[i] < 0) nums[i] = -nums[i];
        }

        return -1;
    }
};