feat: add swift implementation to lcof problem: No.51 (#2936)

Author: klever34

lcof/面试题51. 数组中的逆序对/README.md

@@ -312,6 +312,63 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    private var nums: [Int] = []
+    private var temp: [Int] = []
+
+    func reversePairs(_ nums: [Int]) -> Int {
+        self.nums = nums
+        let n = nums.count
+        self.temp = [Int](repeating: 0, count: n)
+        return mergeSort(0, n - 1)
+    }
+
+    private func mergeSort(_ left: Int, _ right: Int) -> Int {
+        if left >= right {
+            return 0
+        }
+        let mid = (left + right) / 2
+        var count = mergeSort(left, mid) + mergeSort(mid + 1, right)
+        var i = left
+        var j = mid + 1
+        var k = left
+
+        while i <= mid && j <= right {
+            if nums[i] <= nums[j] {
+                temp[k] = nums[i]
+                i += 1
+            } else {
+                count += mid - i + 1
+                temp[k] = nums[j]
+                j += 1
+            }
+            k += 1
+        }
+
+        while i <= mid {
+            temp[k] = nums[i]
+            i += 1
+            k += 1
+        }
+
+        while j <= right {
+            temp[k] = nums[j]
+            j += 1
+            k += 1
+        }
+
+        for i in left...right {
+            nums[i] = temp[i]
+        }
+
+        return count
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题51. 数组中的逆序对/Solution.swift

@@ -0,0 +1,52 @@
+class Solution {
+    private var nums: [Int] = []
+    private var temp: [Int] = []
+
+    func reversePairs(_ nums: [Int]) -> Int {
+        self.nums = nums
+        let n = nums.count
+        self.temp = [Int](repeating: 0, count: n)
+        return mergeSort(0, n - 1)
+    }
+
+    private func mergeSort(_ left: Int, _ right: Int) -> Int {
+        if left >= right {
+            return 0
+        }
+        let mid = (left + right) / 2
+        var count = mergeSort(left, mid) + mergeSort(mid + 1, right)
+        var i = left
+        var j = mid + 1
+        var k = left
+        
+        while i <= mid && j <= right {
+            if nums[i] <= nums[j] {
+                temp[k] = nums[i]
+                i += 1
+            } else {
+                count += mid - i + 1
+                temp[k] = nums[j]
+                j += 1
+            }
+            k += 1
+        }
+        
+        while i <= mid {
+            temp[k] = nums[i]
+            i += 1
+            k += 1
+        }
+        
+        while j <= right {
+            temp[k] = nums[j]
+            j += 1
+            k += 1
+        }
+        
+        for i in left...right {
+            nums[i] = temp[i]
+        }
+        
+        return count
+    }
+}