reentrancy.sol

pragma solidity ^0.6.0;

import 'https://github.com/ConsenSysMesh/openzeppelin-solidity/blob/master/contracts/math/SafeMath.sol';

contract Reentrance {
  
  using SafeMath for uint256;
  mapping(address => uint) public balances;

  function donate(address _to) public payable {
    balances[_to] = balances[_to].add(msg.value); // okay, to make the balance[msg.sender] >= amount, we need to donate to this contract first, then create the fallback function to attack.
  }

  function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
  }

  function withdraw(uint _amount) public {
    if(balances[msg.sender] >= _amount) { // but here it has the if statement, we need to bypass this first.
      (bool result, bytes memory data) = msg.sender.call.value(_amount)(""); // it uses the low-level function call -> this triggers fallback fucntion
      if(result) {
        _amount;
      }
      balances[msg.sender] -= _amount;
    }
  }

   function balanceOfContract() public view returns(uint){
       return address(this).balance;
   }
   
  fallback() external payable {}
}