Java链表maxElement递归

Question

I have been working on an assignment for awhile now one of the methods I have to define is a recursive solution to finding the max element in the list. I feel like my solution is close, but I keep getting the last element inserted back, instead of the max element. Can someone point out what I need to do to solve this? * I have been instructed not to use pre-built classes or methods. *

        int maxElement () {

    if (head == null) {
        return -1;
    }
    else {
        Node current = head;
        return maxElement (current);
    }
}

private int maxElement (Node current) {     
    Node max;
    // If our next node does not exist, our current node has to be max
    if (current.getNext() == null) {
        return current.getKey();
    }
    //  
    else {  

        if (current.getNext().getKey() > current.getKey()) {
            // Assign the larger key as our new max
            max = current.getNext();
            return maxElement (max);
        }
        else {

        }
    }
    return maxElement (max.getNext());
}

Answer 1

You have to keep track of the current max in your recursion:

public Node max() {
    return maxAux(root, null);
}

private Node maxAux(Node current, Node currentMax) {
    if(current == null) {
        return currentMax;
    }
    if(currentMax == null || current.getKey() > currentMax.getKey()) {
        return maxAux(current.getNext(), current);   // current is the new max
    } else {
        return maxAux(current.getNext(), currentMax);
    }
}

Answer 2

Here is a fairly elegant solution:

public Node max(Node node1, Node node2) {
    if (node1 == null || node2 == null)
        return node1;
    else
        return max(node1.getKey() > node2.getKey() ? node1 : node2, node2.getNext());
}

It is called using max(head, head.getNext()) .

If you specifically need to avoid passing the current maximum into the method then:

private static Node currentMax;

private Node maxElement(Node node) {
    if (currentMax == null || node == null)
        return currentMax;
    else if (node.getKey() > currentMax.getKey())
        currentMax = node;
    return maxElement(node.getNext());
}

This is called with currentMax = head; then maxElement(head) .

Answer 3

private Node head, max;

int maxElement () {

    if (head == null) {
        return -1;
    }
    else {
        Node current = head;
        return maxElement (current);
    }
}

    private int maxElement (Node current) {     

        // If our next node does not exist, our current node has to be max
        if (current.getNext() == null) {
            current = this.max;
            return this.max.getKey();
        }
        //  
        else {  

        if (current.getNext().getKey() > current.getKey()) {
            // Assign the larger key as our new max
            this.max = current.getNext();
            return maxElement (current.getNext());
        }
        else {

        }
    }
    return maxElement (max.getNext());
}