feat: add swift implementation to lcp problem: No.28 (#3775)

lcp/LCP 28. 采购方案/README.md

@@ -123,6 +123,31 @@ func purchasePlans(nums []int, target int) int {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func purchasePlans(_ nums: [Int], _ target: Int) -> Int {
+        let mod = 1_000_000_007
+        let nums = nums.sorted()
+        var res = 0
+        var i = 0
+        var j = nums.count - 1
+
+        while i < j {
+            if nums[i] + nums[j] > target {
+                j -= 1
+            } else {
+                res = (res + j - i) % mod
+                i += 1
+            }
+        }
+
+        return res
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcp/LCP 28. 采购方案/Solution.swift

@@ -0,0 +1,20 @@
+class Solution {
+    func purchasePlans(_ nums: [Int], _ target: Int) -> Int {
+        let mod = 1_000_000_007
+        let nums = nums.sorted()
+        var res = 0
+        var i = 0
+        var j = nums.count - 1
+        
+        while i < j {
+            if nums[i] + nums[j] > target {
+                j -= 1
+            } else {
+                res = (res + j - i) % mod
+                i += 1
+            }
+        }
+        
+        return res
+    }
+}