剑指 Offer 36. 二叉搜索树与双向链表

题目描述

输入一棵二叉搜索树，将该二叉搜索树转换成一个排序的循环双向链表。要求不能创建任何新的节点，只能调整树中节点指针的指向。

解题思路

主要是分治的思想，值得注意的是ResultType我们返回一个头指针和一个尾指针。

class Solution {
    static class ResultType {
        Node first;
        Node last;

        public ResultType(Node first, Node last) {
            this.first = first;
            this.last = last;
        }
    }

    public Node treeToDoublyList(Node root) {
        if (root == null) return null;

        ResultType result = helper(root);
        result.last.right = result.first;
        result.first.left = result.last;
        return result.first;
    }

    public ResultType helper(Node root) {
        if (root == null) return null;

        ResultType left = helper(root.left);
        ResultType right = helper(root.right);
        ResultType result = new ResultType(null, null);

        if (left == null) {
            result.first = root;
        } else {
            result.first = left.first;
            left.last.right = root;
            root.left = left.last;
        }

        if (right == null) {
            result.last = root;
        } else {
            result.last = right.last;
            root.right = right.first;
            right.first.left = root;
        }

        return result;
    }
}