二叉树相关题

二叉树的最低公共祖先

中序遍历的后继

• 有右子树 则是右子树上最左节点
• 没有右子树 后继Y 符合左树中最右的节点是x ( 从当前节点 一直往上找 是不是父节点的左子树 是就找到 不是继续往上 找到根了都找不到 那就没有后继 )

    static Node gethouji(Node node) {
        if (node.right != null) {//有右子树 就找右子树中最左的节点
            node = getrightleft(node);
            return node;
        } else {//没有右子树 就看当前节点是不是父节点的 左孩子节点 
//是 就返回结果 不是就继续找上一个儿子节点
            Node parent = node.parent;
            while (parent != null && parent.right == node) {
                node = parent;
                parent = node.parent;
            }
            if (parent == null) {
                return null;
            }
            if (parent.left == node) {
                return parent;
            }
        }
        return null;

    }


    static Node getrightleft(Node node) {
        node = node.right;
        while (node.left != null) {
            node = node.left;
        }
        return node;
    }


  static class Node {
        public int value;
        public Node left;
        public Node right;
        public Node parent;

        public Node(int data) {
            this.value = data;
        }
    }

二叉树的序列化和反序列化

(中序 或者后序 还有按层序列化)

package tree;

import java.util.LinkedList;

public class serializetree {
    static class Node {
        public int value;
        public Node left;
        public Node right;

        public Node(int data) {
            this.value = data;
        }
    }


    static StringBuffer serialize(Node node) {
        if (node == null) {
            return new StringBuffer("#_"); //basecase
        }
        StringBuffer str = new StringBuffer(node.value + "_");  //当前节点去构成串
        StringBuffer sb1 = serialize(node.left);
        StringBuffer sb2 = serialize(node.right);
        return str.append(sb1).append(sb2);//左边加右边 然后按照根左右的顺序拼接
    }


    static Node reserialize(StringBuffer str) {
        String[] strings = str.toString().split("_");
        LinkedList queue = new LinkedList();
        for (int i = 0; i < strings.length; i++) {
            queue.add(strings[i]);
        }
        Node preserialize = preserialize(queue);//进入递归去
        return preserialize;
    }

    static Node preserialize(LinkedList queue) {
        String str = queue.poll();//弹出节点
        if (str.equals("#")) {//#号就不用构建节点了
            return null;
        }
        Node node = new Node(Integer.valueOf(str));
        node.left = preserialize(queue);//构建左右树去拼接当前的节点 
        node.right = preserialize(queue);
        return node;//返回当前节点
    }

    public static void main(String[] args) {
        Node head1 = new Node(1);
        Node head2 = new Node(2);
        Node head3 = new Node(3);
        Node head4 = new Node(4);
        Node head5 = new Node(5);
        head1.left = head2;
        head1.right = head3;
        head2.left = head4;
        head2.right = head5;
        StringBuffer sb = serialize(head1);
        System.out.println(sb);
        Node head = reserialize(sb);
        StringBuffer sb2 = serialize(head);
        System.out.println("反序列化之后------");
        System.out.println(sb2);
    }


}

按层遍历的序列化和反序列化

    public static String serialByLevel(Node head) {
        if (head == null) {
            return "#!";
        }
        String res = head.value + "!";
        Queue queue = new LinkedList();
        queue.offer(head);
        while (!queue.isEmpty()) {
            head = queue.poll();
            if (head.left != null) {
                res += head.left.value + "!";
                queue.offer(head.left);
            } else {
                res += "#!";//为空就加入#!
            }
            if (head.right != null) {
                res += head.right.value + "!";
                queue.offer(head.right);
            } else {
                res += "#!";//为空就加入#!
            }
        }
        return res;
    }

    public static Node reconByLevelString(String levelStr) {
        String[] values = levelStr.split("!");
        int index = 0;
        Node head = generateNodeByString(values[index++]);
        Queue queue = new LinkedList();
        if (head != null) {
            queue.offer(head);
        }
        Node node = null;
        while (!queue.isEmpty()) {
            node = queue.poll();
            node.left = generateNodeByString(values[index++]);
// 把字符串加进去 搞节点出来并作为当前节点的子节点
            node.right = generateNodeByString(values[index++]);
// 把字符串加进去 搞节点出来并作为当前节点的子节点
            if (node.left != null) {
                queue.offer(node.left);
            }
            if (node.right != null) {
                queue.offer(node.right);
            }
        }
        return head;
    }

    public static Node generateNodeByString(String val) {
        if (val.equals("#")) {
            return null;
        }
        return new Node(Integer.valueOf(val));
    }