Pancake的交易对都是从Factory合约创建出来的，也可以从Factory合约获取，Facotory合约代码在这里

https://github.com/pancakeswap/pancake-swap-core/blob/master/contracts/PancakeFactory.sol

关键代码：

mapping(address => mapping(address => address)) public getPair;
    address[] public allPairs;

    event PairCreated(address indexed token0, address indexed token1, address pair, uint);

    constructor(address _feeToSetter) public {
        feeToSetter = _feeToSetter;
    }

    function allPairsLength() external view returns (uint) {
        return allPairs.length;
    }

    function createPair(address tokenA, address tokenB) external returns (address pair) {
        require(tokenA != tokenB, 'Pancake: IDENTICAL_ADDRESSES');
        (address token0, address token1) = tokenA < tokenB ? (tokenA, tokenB) : (tokenB, tokenA);
        require(token0 != address(0), 'Pancake: ZERO_ADDRESS');
        require(getPair[token0][token1] == address(0), 'Pancake: PAIR_EXISTS'); // single check is sufficient
        bytes memory bytecode = type(PancakePair).creationCode;
        bytes32 salt = keccak256(abi.encodePacked(token0, token1));
        assembly {
            pair := create2(0, add(bytecode, 32), mload(bytecode), salt)
        }
        IPancakePair(pair).initialize(token0, token1);
        getPair[token0][token1] = pair;
        getPair[token1][token0] = pair; // populate mapping in the reverse direction
        allPairs.push(pair);
        emit PairCreated(token0, token1, pair, allPairs.length);
    }

从上面的代码可见，allPairs就是所有的交易对，另外，也可以通过PairCreated事件获取。