ERC-721 NFT Standard

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SimpleNFT {
    string public name   = "MyNFT";
    string public symbol = "MNFT";

    uint256 private _nextTokenId;

    mapping(uint256 => address) public ownerOf;
    mapping(address => uint256) public balanceOf;
    mapping(uint256 => address) public getApproved;
    mapping(uint256 => string)  private _tokenURIs;

    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);
    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);

    function mint(address to, string memory uri) public returns (uint256 id) {
        id = _nextTokenId++;
        ownerOf[id]    = to;
        _tokenURIs[id] = uri;
        balanceOf[to]++;
        emit Transfer(address(0), to, id);
    }

    function tokenURI(uint256 id) public view returns (string memory) {
        require(ownerOf[id] != address(0), "Nonexistent token");
        return _tokenURIs[id];
    }

    function transferFrom(address from, address to, uint256 id) public {
        require(ownerOf[id] == from, "Not owner");
        require(msg.sender == from || msg.sender == getApproved[id],
            "Not authorized");
        ownerOf[id] = to;
        balanceOf[from]--;
        balanceOf[to]++;
        delete getApproved[id];
        emit Transfer(from, to, id);
    }

    function approve(address to, uint256 id) public {
        require(msg.sender == ownerOf[id], "Not owner");
        getApproved[id] = to;
        emit Approval(msg.sender, to, id);
    }
}