This is the solver for FinTech 2022 Homework 4 Problem 4

Solver.sol

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

contract Solver {

    bytes32[] public hashes;
    bytes32[] public proof;

    constructor() {

        string[7] memory tx = [
            "zkpenguin",
            "zkpancake",
            "zkpolice",
            "zkpig",
            "zkplayground",
            "zkpigeon",
            "zkpoison"
        ];

        for (uint256 i = 0; i < tx.length; i++) {
            hashes.push(keccak256(abi.encodePacked(tx[i])));
        }
    }

    function computeProof() public {
        // 1st hash
        proof.push(hashes[0]);
        // 2nd hash
        proof.push(_hashPair(hashes[1], hashes[2]));
        // 3rd hash
        proof.push(_hashPair(_hashPair(hashes[3], hashes[5]), hashes[6])); // maybe add keccak256 here
    }

    function _hashPair(bytes32 a, bytes32 b) private pure returns (bytes32) {
        return a < b ? _efficientHash(a, b) : _efficientHash(b, a);
    }

    function _efficientHash(bytes32 a, bytes32 b)
        private
        pure
        returns (bytes32 value)
    {
        assembly {
            mstore(0x00, a)
            mstore(0x20, b)
            value := keccak256(0x00, 0x40)
        }
    }
}