[156. Binary Tree Upside Down] #leetcode

156_0902.java

class Solution {
    public TreeNode upsideDownBinaryTree(TreeNode root) {
        if (root == null) return null;
        
        TreeNode node = root.left;
        TreeNode right = root.right;
        TreeNode preNode = root;
        root.left = null;
        root.right = null;
        
        while (node != null) {
            TreeNode leftLeft = node.left;
            TreeNode leftRight = node.right;
            
            node.left = right;
            node.right = preNode;
            
            right = leftRight;
            preNode = node;
            node = leftLeft;
        }
        return preNode;
    }
}