426. 将二叉搜索树转化为排序的双向链表

English Version

题目描述

将一个 二叉搜索树 就地转化为一个 已排序的双向循环链表 。

对于双向循环列表，你可以将左右孩子指针作为双向循环链表的前驱和后继指针，第一个节点的前驱是最后一个节点，最后一个节点的后继是第一个节点。

特别地，我们希望可以 就地 完成转换操作。当转化完成以后，树中节点的左指针需要指向前驱，树中节点的右指针需要指向后继。还需要返回链表中最小元素的指针。

 

示例 1：

输入：root = [4,2,5,1,3] 


输出：[1,2,3,4,5]

解释：下图显示了转化后的二叉搜索树，实线表示后继关系，虚线表示前驱关系。

示例 2：

输入：root = [2,1,3]
输出：[1,2,3]

示例 3：

输入：root = []
输出：[]
解释：输入是空树，所以输出也是空链表。

示例 4：

输入：root = [1]
输出：[1]

 

提示：

• -1000 <= Node.val <= 1000
• Node.left.val < Node.val < Node.right.val
• Node.val 的所有值都是独一无二的
• 0 <= Number of Nodes <= 2000

解法

• 排序链表：二叉搜索树中序遍历得到有序序列
• 循环链表：头节点指向链表尾节点，尾节点指向链表头节点
• 双向链表：prev.right = cur、cur.left = prev、prev = cur

Python3

"""
# Definition for a Node.
class Node:
    def __init__(self, val, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
"""


class Solution:
    def treeToDoublyList(self, root: 'Optional[Node]') -> 'Optional[Node]':
        def dfs(root):
            if root is None:
                return
            nonlocal prev, head
            dfs(root.left)
            if prev:
                prev.right = root
                root.left = prev
            else:
                head = root
            prev = root
            dfs(root.right)

        if root is None:
            return None
        head = prev = None
        dfs(root)
        prev.right = head
        head.left = prev
        return head

Java

/*
// Definition for a Node.
class Node {
    public int val;
    public Node left;
    public Node right;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val,Node _left,Node _right) {
        val = _val;
        left = _left;
        right = _right;
    }
};
*/

class Solution {
    private Node prev;
    private Node head;

    public Node treeToDoublyList(Node root) {
        if (root == null) {
            return null;
        }
        prev = null;
        head = null;
        dfs(root);
        prev.right = head;
        head.left = prev;
        return head;
    }

    private void dfs(Node root) {
        if (root == null) {
            return;
        }
        dfs(root.left);
        if (prev != null) {
            prev.right = root;
            root.left = prev;
        } else {
            head = root;
        }
        prev = root;
        dfs(root.right);
    }
}

C++

/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;

    Node() {}

    Node(int _val) {
        val = _val;
        left = NULL;
        right = NULL;
    }

    Node(int _val, Node* _left, Node* _right) {
        val = _val;
        left = _left;
        right = _right;
    }
};
*/

class Solution {
public:
    Node* prev;
    Node* head;

    Node* treeToDoublyList(Node* root) {
        if (!root) return nullptr;
        prev = nullptr;
        head = nullptr;
        dfs(root);
        prev->right = head;
        head->left = prev;
        return head;
    }

    void dfs(Node* root) {
        if (!root) return;
        dfs(root->left);
        if (prev)
        {
            prev->right = root;
            root->left = prev;
        }
        else head = root;
        prev = root;
        dfs(root->right);
    }
};

Go

/**
 * Definition for a Node.
 * type Node struct {
 *     Val int
 *     Left *Node
 *     Right *Node
 * }
 */

func treeToDoublyList(root *Node) *Node {
	if root == nil {
		return root
	}
	var prev, head *Node

	var dfs func(root *Node)
	dfs = func(root *Node) {
		if root == nil {
			return
		}
		dfs(root.Left)
		if prev != nil {
			prev.Right = root
			root.Left = prev
		} else {
			head = root
		}
		prev = root
		dfs(root.Right)
	}
	dfs(root)
	prev.Right = head
	head.Left = prev
	return head
}

JavaScript

/**
 * // Definition for a Node.
 * function Node(val, left, right) {
 *      this.val = val;
 *      this.left = left;
 *      this.right = right;
 *  };
 */

/**
 * @param {Node} root
 * @return {Node}
 */
var treeToDoublyList = function (root) {
    if (!root) return root;
    let prev = null;
    let head = null;

    function dfs(root) {
        if (!root) return;
        dfs(root.left);
        if (prev) {
            prev.right = root;
            root.left = prev;
        } else {
            head = root;
        }
        prev = root;
        dfs(root.right);
    }
    dfs(root);
    prev.right = head;
    head.left = prev;
    return head;
};

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