题解 | #二叉搜索树与双向链表#

import java.util.*;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    public TreeNode Convert(TreeNode root) {
        if (root == null) {
            return null;
        }
        List<TreeNode> list = inOrder(root);
        TreeNode head = list.get(0);
        head.left = null;
        TreeNode pre = head;
        for (int i = 1; i < list.size(); i++) {
            TreeNode cur = list.get(i);
            pre.right = cur;
            cur.left = pre;
            pre = cur;
        }
        list.get(list.size() - 1).right = null;
        return head;
    }

    private List<TreeNode> inOrder(TreeNode root) {
        List<TreeNode> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while (!stack.isEmpty() || cur != null) {
            if (cur != null) {
                stack.push(cur);
                cur = cur.left;
            } else {
                cur = stack.pop();
                res.add(cur);
                cur = cur.right;
            }
        }
        return res;
    }
}