Merge pull request soapyigu#190 from soapyigu/LinkedList

[LinkedList] Add a solution to Flatten Binary Tree to Linked List

LinkedList/FlattenBinaryTreeLinkedList.swift

@@ -0,0 +1,47 @@
+/**
+ * Question Link: https://leetcode.com/problems/flatten-binary-tree-to-linked-list/
+ * Primary idea: Reset left to nil and change current node to left child every time
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.left = nil
+ *         self.right = nil
+ *     }
+ * }
+ */
+
+class FlattenBinaryTreeLinkedList {
+    func flatten(_ root: TreeNode?) {
+        helper(root)
+    }
+
+    private func helper(_ node: TreeNode?) -> TreeNode? {
+        var node = node
+        if node == nil {
+            return node
+        }
+        if node!.left == nil && node!.right == nil {
+            return node
+        }
+
+        let left = node!.left, right = node!.right
+        node!.left = nil
+
+        if let left = left {
+            node!.right = left
+            node = helper(left)
+        }
+        if let right = right {
+            node!.right = right
+            node = helper(right)
+        }
+
+        return node
+    }
+}