题解 | #二叉树展开为单链表#

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 {
    /**
     * 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
     *
     * 
     * @param root TreeNode类 
     */
    public void expandTree(TreeNode root) {
        // write code here
        if (null == root || (null == root.left && null == root.right)) {
            return;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode node = root;
        TreeNode previousNode = root;
        stack.push(node);
        while (!stack.isEmpty()) {
            node = stack.pop();
            if (null != node.right) {
                stack.push(node.right);
            }
            if (null != node.left) {
                stack.push(node.left);
            }
            if (node != previousNode) {
                previousNode.right = node;
                previousNode = node;
            }
            previousNode.left = null;
        }
        previousNode.right = null;
    }
}