学习1：调用合约接口（授权，提供流动性）_0x7a250d5630b4cf539739df2c5dacb4c659f2488d

uniswap 官方接口文档：https://uniswap.org/docs/v2/smart-contracts/router02/#interface

测试地址：0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.2;


// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.2;

// helper methods for interacting with ERC20 tokens and sending ETH that do not consistently return true/false
library TransferHelper {
   
    function safeApprove(address token, address to, uint value) internal {
   
        // bytes4(keccak256(bytes('approve(address,uint256)')));
        (bool success, bytes memory data) = token.call(abi.encodeWithSelector(0x095ea7b3, to, value));
        require(success && (data.length == 0 || abi.decode(data, (bool))), 'TransferHelper: APPROVE_FAILED');
    }

    function safeTransfer(address token, address to, uint value) internal {
   
        // bytes4(keccak256(bytes('transfer(address,uint256)')));
        (bool success, bytes memory data) = token.call(abi.encodeWithSelector(0xa9059cbb, to, value));
        require(success && (data.length == 0 || abi.decode(data, (bool))), 'TransferHelper: TRANSFER_FAILED');
    }

    function safeTransferFrom(address token, address from, address to, uint value) internal {
   
        // bytes4(keccak256(bytes('transferFrom(address,address,uint256)')));
        (bool success, bytes memory data) = token.call(abi.encodeWithSelector(0x23b872dd, from, to, value));
        require(success && (data.length == 0 || abi.decode(data, (bool))), 'TransferHelper: TRANSFER_FROM_FAILED');
    }

    function safeTransferETH(address to, uint value) internal {
   
        (bool success,) = to.call{
   value:value}(new bytes(0));
        require(success, 'TransferHelper: ET
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