题解 | #牛群的树形结构展开#

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 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *先序 根->左->右
	 适用栈进行先序遍历，依次添加新建节点的右子树
     *
     * @param root TreeNode类
     * @return TreeNode类
     */
    public TreeNode flattenTree (TreeNode root) {
        TreeNode  res = new TreeNode(-1);
        TreeNode index = res;
        Stack<TreeNode> stack =  new Stack<>();
        while (!stack.isEmpty() || root != null) {
            if (root != null) {
                stack.push(root);
                index.right = new TreeNode(root.val);
                index = index.right;
                root = root.left;
                

            } else {
                TreeNode node = stack.pop();
                if (null != node.right) {
                    root = node.right;
                }
            }
        }
        return res.right;
    }
}