ERC20.sol

pragma solidity ^0.4.25;

import "./SafeMath.sol";
import "./IERC20.sol";

contract ERC20 is IERC20 {
    using SafeMath for uint256;
    
    string public constant name = "haha token";
    uint8 public constant decimals = 18;
    string public constant symbol = "HHT";
    
    uint256 private _totalSupply;
    mapping(address => uint256) _balances;
    mapping(address => mapping(address => uint256)) _approve;
    
    // 所有存在的 Token 數量
    function totalSupply() external view returns (uint256) {
        return _totalSupply;
    }
    
    // 讀取 tokenOwner 這個 address 所持有的 Token 數量
    // address => uint256
    function balanceOf(address tokenOwner) external view returns (uint256 balance) {
        return _balances[tokenOwner];
    }
    
    // 從 msg.sender 轉 tokens 個 Token 給 to 這個 address
    // msg.sender ---tokens---> to 
    function transfer(address to, uint256 tokens) external returns (bool success) {
        return _transfer(msg.sender, to, tokens);
    }
    
    // 得到 tokenOwner 授權給 spender 使用的 Token 剩餘數量
    function allowance(address tokenOwner, address spender) external view returns (uint256 remaining) {
        return _approve[tokenOwner][spender];
    }
  
    // tokenOwner -> spender -> tokens
    // address => address => uint256
    // msg.sender 授權給 spender 可使用自己的 tokens 個 Token
    function approve(address spender, uint256 tokens) external returns (bool success) {
        _approve[msg.sender][spender] = tokens;
        emit Approval(msg.sender, spender, tokens);
        return true;
    }

    // 將 tokens 個 Token 從 from 轉到 to
    function transferFrom(address from, address to, uint256 tokens) external returns (bool success) {
        _approve[from][msg.sender] = _approve[from][msg.sender].sub(tokens);
        
        return _transfer(from, to, tokens);
    }
    
    function _transfer(address from, address to, uint256 tokens) internal returns (bool success) {
        _balances[from] = _balances[from].sub(tokens);
        _balances[to] = _balances[to].add(tokens);
        emit Transfer(from, to, tokens);
        return true;
    }
}

ERC20Interface.sol

pragma solidity ^0.4.25;

interface IERC20 {
    
  // 所有存在的 Token 數量
  function totalSupply() external view returns (uint256);
  
  // 讀取 tokenOwner 這個 address 所持有的 Token 數量
  function balanceOf(address tokenOwner) external view returns (uint256 balance);
  
  // 從 msg.sender 轉 tokens 個 Token 給 to 這個 address
  function transfer(address to, uint256 tokens) external returns (bool success);
  
  
  // 得到 tokenOwner 授權給 spender 使用的 Token 剩餘數量
  function allowance(address tokenOwner, address spender)
    external view returns (uint256 remaining);
  
  // msg.sender 授權給 spender 可使用自己的 tokens 個 Token
  function approve(address spender, uint256 tokens)
    external returns (bool success);

  // 將 tokens 個 Token 從 from 轉到 to
  function transferFrom(address from, address to, uint256 tokens)
    external returns (bool success);

  event Transfer(
    address indexed from,
    address indexed to,
    uint256 tokens
  );

  event Approval(
    address indexed owner,
    address indexed spender,
    uint256 tokens
  );
}

SafeMath.sol

pragma solidity ^0.4.25;

library SafeMath {

  function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a * b;
    require(c / a == b);

    return c;
  }

  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b > 0); // Solidity only automatically asserts when dividing by 0
    uint256 c = a / b;
    
    return c;
  }

  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b <= a); // underflow 
    uint256 c = a - b;

    return c;
  }

  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    require(c >= a); // overflow

    return c;
  }

  function mod(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b != 0);
    return a % b;
  }
}

contract Main {
    function test() public pure returns (uint256) {
        uint256 a = 100;
        uint256 b = 10;
        return SafeMath.add(a, b);
    }
}