Merge pull request #216 from Desgard/master

Add solution for Merge Two Binary Trees

Author: soapyigu

README.md

@@ -27,7 +27,7 @@
 * [Microsoft](#microsoft)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 500+ questions. Currently we have 219 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them. Thank you for great contributions from [CharleneJiang](https://github.com/CharleneJiang), [ReadmeCritic](https://github.com/ReadmeCritic), [demonkoo](https://github.com/demonkoo), [DaiYue](https://github.com/DaiYue), [Quaggie](https://github.com/Quaggie) and [jindulys](https://github.com/jindulys).
+[Problem Status](#problem-status) shows the latest progress to all 500+ questions. Currently we have 221 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them. Thank you for great contributions from [CharleneJiang](https://github.com/CharleneJiang), [ReadmeCritic](https://github.com/ReadmeCritic), [demonkoo](https://github.com/demonkoo), [DaiYue](https://github.com/DaiYue), [Quaggie](https://github.com/Quaggie) and [jindulys](https://github.com/jindulys).
 
 
 ## Array
@@ -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 |
@@ -275,6 +276,7 @@
 [Meeting Rooms II](https://leetcode.com/problems/meeting-rooms-ii/)| [Swift](./Sort/MeetingRoomsII.swift)| Medium| O(nlogn)| O(n)|
 [Merge Intervals](https://leetcode.com/problems/merge-intervals/)| [Swift](./Sort/MergeIntervals.swift)| Hard| O(nlogn)| O(n)|
 [Alien Dictionary](https://leetcode.com/problems/alien-dictionary/)| [Swift](./Sort/AlienDictionary.swift)| Hard| O(nm)| O(nm)|
+[Array Partition I](https://leetcode.com/problems/array-partition-i/description/)| [Swift](./Sort/ArrayPartitionI.swift)|Easy| O(nlogn)| O(n)|
 
 ## Union Find
 | Title | Solution | Difficulty | Time | Space |

Sort/ArrayPartitionI.swift

@@ -0,0 +1,21 @@
+class ArrayPartitionI {
+    func arrayPairSum(_ nums: [Int]) -> Int {
+        var arr = nums
+        arr = arr.sorted()
+        var res = 0
+        for i in 0 ..< arr.count {
+            if i & 1 != 0 {
+                res += min(arr[i], arr[i - 1])
+            }
+        }
+        return res
+    }
+}
+
+class ArrayPartitionI_2 {
+    func arrayPairSum(_ nums: [Int]) -> Int {
+        return nums.sorted(by: <).enumerated()
+            .flatMap { $0 % 2 == 0 ? $1 : nil }
+            .reduce(0) { $0 + $1 }
+    }
+}

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 MergeTwoBinaryTrees {
+    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
+    }
+}