Get a different number - peer code review

GettingADifferentNumber_peerCode.cpp

#include <iostream>
#include <vector>

using namespace std;

int getDifferentNumber( const vector<int>& arr ) 
{   
  for(int i = 0; i < arr.size(); i++){
    if(Math.Abs(arr[i]) < arr.size()){      
      arr[arr[i]] = -arr[arr[i]];
    }
  }  
  
  for(int i = 0;i < arr.size(); i++){
     if(arr[i]>=0)
     {
       return i;
     }
  }
  
  return arr.size();
}

int main() {  
  vector<int> input{1, 3, 2, 5};
  int ret=getDifferentNumber(input);
  
  cout<<ret<<endl;
  return 0;
}

/*
[-2,-3,-5,1,3,4,5,6,7,8,9]

[]  6

[1, 3, 2, 5]   -> 0

arr[1]=-arr[1]

[1, 3, 2, 0, 100] -> 4

array int[] visit -> only save the number if number < arr's length 

0 -> visit[0] = 1
4 -> visit[4] = 1

[0,10000]
size of 10000 array 
*/
// first to find the min number of this array, then ,keep increment this number until i got a 
// number that not belongs to this array, 
// creat an array with the same size of the input array as arrray tmp, and for each element a, 
// if this element is less than the size of the input array, and then i just flag the tmp[a]. 
// after that , i iterate tmp array from begining to end , to find the first that did not be 
// flagged. this index is the one we need.