Add the solution of 561. Array Partition I

Author: Desgard

README.md

@@ -277,6 +277,7 @@
 [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 +1,13 @@
+class Solution {
+    func arrayPairSum(_ nums: [Int]) -> Int {
+        var arr = nums
+        arr = arr.sorted { $0 < $1 }
+        var res = 0
+        for i in 0 ..< arr.count {
+            if i & 1 != 0 {
+                res += min(arr[i], arr[i - 1])
+            }
+        }
+        return res
+    }
+}