[leetcode] Binary Tree Upside Down

做这题的一个提升就是 先把recursive call当作能work的api.. 解决小问题先，再看小节 然后大问题其实就解决了

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/** * Definition for a binary tree node. * 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 || root.left == null){ return root; } TreeNode one = upsideDownBinaryTree(root.right); TreeNode two = upsideDownBinaryTree(root.left); root.left.left = one; root.left.right = root; root.left = null; root.right = null; return two; } }