A simple address registry contract for Ethereum

registry.sol

pragma solidity >=0.4.22 <0.6.0;

contract registry {
    address[] public addresses;
    mapping(address => bool) public contains;
    
    function register() public {
        require(!contains[msg.sender]);
        contains[msg.sender] = true;
        addresses.push(msg.sender);
    }
    
    function getSize() public view returns (uint) {
        return addresses.length;
    }
    
    function getWinner(uint index) public view returns (address) {
        return addresses[index];
    }
}