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LeetCode-156-Binary-Tree-Upside-Down.java
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LeetCode-156-Binary-Tree-Upside-Down.java
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/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
// 1. Recursive
// public TreeNode upsideDownBinaryTree(TreeNode root) {
// if (root == null || root.left == null) return root;
// TreeNode newRoot = upsideDownBinaryTree(root.left);
// root.left.left = root.right;
// root.left.right = root;
// root.left = null;
// root.right = null;
// return newRoot;
// }
// 2.Iterative
/*
The idea is the move the tree down to its left child.
Record prev node, and prev node's right child.
curr.left = prev node's right child
curr.right = prev
*/
public TreeNode upsideDownBinaryTree(TreeNode root) {
Stack<TreeNode> stack = new Stack<>();
TreeNode curr = root, prev = null, prevRight = null;
while(curr != null) {
TreeNode nextLeft = curr.left;
// swapping nodes now
curr.left = prevRight;
prevRight = curr.right;
curr.right = prev;
// move to next left node
prev = curr;
curr = nextLeft;
}
return prev;
}
}