https://oj.leetcode.com/problems/binary-tree-upside-down/

http://blog.csdn.net/xudli/article/details/42362441 

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode UpsideDownBinaryTree(TreeNode root)
    {
        if (root == null)
            return null;
        
        Stack stack = new Stack<>();
        TreeNode node = root;
        while (node != null)
        {
            stack.push(node);
            node = node.left;
        }
        
        TreeNode newhead = null;
        while (!stack.empty())
        {
            node = stack.pop();
            if (newhead == null)
                newhead = node;
                
            if (!stack.empty())
            {
                node.left = stack.peek().right;
                node.right = stack.peek();
            }
            else
            {
                node.left = null;
                node.right = null;
            }
        }
        
        return newhead;
    }
}