CookieMonster.sol

pragma solidity ^0.4.24;

contract CookieMonster {

  struct Cookies {
      uint num;
      bool exists;
  }   
  
  mapping (address => Cookies) map;
  address[] addresses;
  
  function addCookies(uint number) public {
    if (!map[msg.sender].exists) {
        addresses.push(msg.sender);
        map[msg.sender].exists = true;
    }
    
    map[msg.sender].num += number;
  }

  function numCookies() public view returns (uint) {
    uint sum = 0;
    for(uint i=0; i<addresses.length; ++i) {
      sum += map[addresses[i]].num;
    }
    
    return sum;
  }
}