287. Find the Duplicate Number

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
      int n = nums.size(), ans = 0;
      
      for(int i=0; i<n; i++)
      {
          int index = abs(nums[i])-1;
          if(nums[index] > 0)
          {
              nums[index] = -1 * nums[index];
          }
          else
          {
              ans = index + 1;
          }
      }
      return ans;

    }
};