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public class BinaryTreeUpsideDown {

/**

* 将root的左子树看作一个整体，root.left就作为倒置后的root和root.right的新parent

* 注意若root的左子树为空，则无法倒置了，什么也不做

* 注意最后要给root的左右清零

*/

public TreeNode upsideDownBinaryTree(TreeNode root) {

if(root == null || root.left == null) {

return root;

}

TreeNode newRoot = upsideDownBinaryTree(root.left);

root.left.left = root.right; // node 2 left children

root.left.right = root; // node 2 right children

root.left = null;

root.right = null;

return newRoot;

}

}