Create 156-binary-tree-upside-down.md

Author: everthis

detailed/156-binary-tree-upside-down.md

@@ -0,0 +1,44 @@
+![alt text](https://github.com/everthis/leetcode-js/blob/master/images/binary-tree-upside-down.webp "binary-tree-upside-down")
+
+```js
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {TreeNode}
+ */
+const upsideDownBinaryTree = function(root) {
+  let curr = root
+  let next = null
+  let temp = null
+  let prev = null
+  while (curr !== null) {
+    next = curr.left
+    curr.left = temp
+    temp = curr.right
+    curr.right = prev
+    prev = curr
+    curr = next
+  }
+  return prev
+}
+
+// another
+
+const upsideDownBinaryTree = function(root) {
+  if (root == null || root.left == null) {
+    return root
+  }
+  const newRoot = upsideDownBinaryTree(root.left)
+  root.left.left = root.right
+  root.left.right = root
+  root.left = null
+  root.right = null
+  return newRoot
+}
+```