Add solution for Merge Two Binary Trees

Desgard · Desgard · commit a4db329eb550 · 2017-08-10T09:48:34.000+08:00
Signed-off-by: Desgard_Duan &lt;gua@desgard.com&gt;
diff --git a/README.md b/README.md
@@ -154,6 +154,7 @@
 [Path Sum III](https://leetcode.com/problems/path-sum-iiI/)| [Swift](./Tree/PathSumIII.swift)| Easy| O(n^2)| O(1)|
 [Unique Binary Search Trees](https://leetcode.com/problems/unique-binary-search-trees/)| [Swift](./Tree/UniqueBinarySearchTrees.swift)| Medium| O(n^2)| O(n)|
 [Recover Binary Search Tree](https://leetcode.com/problems/recover-binary-search-tree/)| [Swift](./Tree/RecoverBinarySearchTree.swift)| Hard| O(n)| O(1)|
+[Merge Two Binary Trees](https://leetcode.com/problems/merge-two-binary-trees/description/) | [Swift](./Tree/MergeTwoBinaryTrees.swift) | Easy | O(n) | O(n) |
 
 ## Dynamic programming
 | Title | Solution | Difficulty | Time | Space |
@@ -188,6 +189,7 @@
 [Burst Ballons](https://leetcode.com/problems/burst-balloons/)| [Swift](./DP/BurstBalloons.swift)| Hard| O(n^3)| O(n)|
 [Frog Jump](https://leetcode.com/problems/frog-jump/)| [Swift](./DP/FrogJump.swift)| Hard| O(n^2)| O(n)|
 
+
 ## Depth-first search
 | Title | Solution | Difficulty | Time | Space |
 | ----- | -------- | ---------- | ---- | ----- |
@@ -767,3 +769,5 @@
 |	[Swift](./String/LongestSubstringWithoutRepeatingCharacters.swift)	|	3	|	[Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/)	| Medium	|
 |	[Swift](./Math/AddTwoNumbers.swift)	|	2	|	[Add Two Numbers](https://leetcode.com/problems/add-two-numbers/)	| Medium	|
 |	[Swift](./Array/TwoSum.swift)	|	1	|	[Two Sum](https://leetcode.com/problems/two-sum/)	| Easy	|
+
+
diff --git a/Tree/MergeTwoBinaryTrees.swift b/Tree/MergeTwoBinaryTrees.swift
@@ -0,0 +1,27 @@
+/**
+ * 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 Solution {
+    func mergeTrees(_ t1: TreeNode?, _ t2: TreeNode?) -> TreeNode? {
+        guard let t1 = t1 else {
+            return t2
+        }
+        guard let t2 = t2 else {
+            return t1
+        }
+        t1.val += t2.val
+        t1.left = mergeTrees(t1.left, t2.left)
+        t1.right = mergeTrees(t1.right, t2.right)
+        return t1
+    }
+}
