pragma solidity ^0.4.24;

contract Auction {
    address public owner;
    address public highestBidder; // The highest bidder's address
    uint public highestBid; // The amount of the highest bid
    mapping(address => uint) public userBalances;	// mapping for the amount to return

    constructor() public
    {
   	 // contractor
   	 // 1. Initialize highest bid and the bidder's address
   	 owner = msg.sender;
   	 highestBidder = 0;
   	 highestBid = 0;
    }

    function bid() payable public
    {
   	 // Funtion to process bid
   	 // 1. Check if the bid is greater than the current highest bid
   	 // 2. Update status variable and the amount to return
   	 if(highestBid < msg.value) {
   		 highestBidder = msg.sender;
   		 highestBid = msg.value;
   		 userBalances[msg.sender] = msg.value;
   	 }
    }

    function withdraw() public {
   	 // Function to withdraw the amount of bid to return
   	 // 1. Check if the amount to return is greater than zero
   	 // 2. Update status variablle and return bid
   	 if(userBalances[msg.sender] > 0) {
   		 msg.sender.transfer(userBalances[msg.sender]);
   		 userBalances[msg.sender] = 0;
   	 }
    }
}