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 pRootOfTree) {
        if(pRootOfTree == null) return null;
        ArrayList<Integer> preList = new ArrayList<>();
        pre(pRootOfTree, preList);
        TreeNode root = new TreeNode(preList.get(0));
        TreeNode preNode = root;
        for(int i = 1; i < preList.size(); i ++){
            TreeNode currentNode = new TreeNode(preList.get(i));
            preNode.right = currentNode;
            currentNode.left = preNode;
            preNode = currentNode;
        }
        return root;
    }
    void pre(TreeNode root, ArrayList<Integer> preList){
        if(root == null) return;
        pre(root.left, preList);
        preList.add(root.val);
        pre(root.right, preList);
    }
}