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Author: BaffinLee

101-200/114. Flatten Binary Tree to Linked List.md

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+# 114. Flatten Binary Tree to Linked List
+
+- Difficulty: Medium.
+- Related Topics: Tree, Depth-first Search.
+- Similar Questions: .
+
+## Problem
+
+Given a binary tree, flatten it to a linked list in-place.
+
+For example, given the following tree:
+
+```
+    1
+   / \
+  2   5
+ / \   \
+3   4   6
+```
+
+The flattened tree should look like:
+
+```
+1
+ \
+  2
+   \
+    3
+     \
+      4
+       \
+        5
+         \
+          6
+```
+
+
+## Solution
+
+```javascript
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {void} Do not return anything, modify root in-place instead.
+ */
+var flatten = function (root) {
+  helper(root);
+};
+
+var helper = function (root) {
+  if (!root) return null;
+
+  var leftLast = helper(root.left);
+  var rightLast = helper(root.right);
+  
+  if (root.left) {
+    leftLast.right = root.right;
+    root.right = root.left;
+  }
+  
+  root.left = null;
+  
+  return rightLast || leftLast || root;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(1).