如何在Java中从右向左遍历二叉树? [英] How can i traverse a binary tree from right to left in java?

问题描述

我想从右到左遍历二叉树，并将姓氏相同的每个项目添加到队列中.我已经正确实现了一个队列列表类和一个树节点类，但是，当我尝试查找某些东西时，我得到了一个空指针异常. (当然，我已经为二叉树写了一个插入方法).

I want to traverse a binary tree from right to left and add to a queue every item with same last name. I have correctly implement a Queue List class and a Tree Node class but, i get a null pointer exception when i try to find something. (Of course i have written an insertion method for the binary tree).

public class ST {


    private TreeNode root;
    private int size;
    private Queue q;


    public Queue searchByLastName(String last_name) {
            searchByLastNameRec(this.root, last_name);
            return q;
        }

        private void searchByLastNameRec(TreeNode newroot, String last_name) {
            if (newroot == null)
                return;
            if (newroot.right != null) {
                if (newroot.right.item.getLast_name().equalsIgnoreCase(last_name)) {
                    q.put(newroot.right.item);
                }
                searchByLastNameRec(newroot.right, last_name);
            }
            if (newroot.left != null) {
                if (newroot.left.item.getLast_name().equalsIgnoreCase(last_name)) {
                    q.put(newroot.left.item);
                }
                searchByLastNameRec(newroot.left, last_name);
            }
        }


public class TreeNode {
    Suspect item;
    TreeNode left, right, parent;
    int N;

    public TreeNode(Suspect item) {
        if (item == null)
            throw new IllegalArgumentException();
        this.item = item;

    }

}

推荐答案

尝试一下

private void searchByLastNameRec(TreeNode newroot, String last_name) {
    if (newroot == null || newroot.item == null)
        return;
    if (Objects.equals(last_name, newroot.item.getLast_name()))
        q.put(newroot.item);
    searchByLastNameRec(newroot.right, last_name);
    searchByLastNameRec(newroot.left, last_name);
}

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