Push and pop to the end

LinkedList.sol

pragma solidity ^0.4.21;

contract LinkedList {
    bool public isEmpty = true;
    uint public length = 0;
    bytes32 public head;
    bytes32 public tail;

    struct LinkNode {
        string value;
        bytes32 next;
        bytes32 prev;
    }

    mapping (bytes32 => LinkNode) public nodes;

    function push(string value) public {
        bytes32 key = keccak256(value, length);
        nodes[key].value = value;

        if (length++ == 0) {
            head = key;
            isEmpty = false;
        } else {
            nodes[tail].next = key;
            nodes[key].prev = tail;
        }
        tail = key;
    }

    function pop() public returns(string) {
        require(length > 0);
        tail = nodes[tail].prev;
        delete nodes[tail].next;
        if (--length == 0) {
            delete nodes[head].next;
            delete head;
            isEmpty = true;
        }
        return nodes[tail].value;
    }
}