feat: add swift implementation to lcof2 problem: No.071 (#3223)

Author: klever34

lcof2/剑指 Offer II 071. 按权重生成随机数/README.md

@@ -222,6 +222,44 @@ func (this *Solution) PickIndex() int {
  */
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    private var presum: [Int]
+
+    init(_ w: [Int]) {
+        let n = w.count
+        presum = [Int](repeating: 0, count: n + 1)
+        for i in 0..<n {
+            presum[i + 1] = presum[i] + w[i]
+        }
+    }
+
+    func pickIndex() -> Int {
+        let n = presum.count
+        let x = Int.random(in: 1...presum[n - 1])
+        var left = 0
+        var right = n - 2
+        while left < right {
+            let mid = (left + right) >> 1
+            if presum[mid + 1] >= x {
+                right = mid
+            } else {
+                left = mid + 1
+            }
+        }
+        return left
+    }
+}
+/**
+ * Your Solution object will be instantiated and called as such:
+ * let w = [1]
+ * let solution = Solution(w)
+ * solution.pickIndex()
+ */
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 071. 按权重生成随机数/Solution.swift

@@ -0,0 +1,33 @@
+class Solution {
+    private var presum: [Int]
+
+    init(_ w: [Int]) {
+        let n = w.count
+        presum = [Int](repeating: 0, count: n + 1)
+        for i in 0..<n {
+            presum[i + 1] = presum[i] + w[i]
+        }
+    }
+
+    func pickIndex() -> Int {
+        let n = presum.count
+        let x = Int.random(in: 1...presum[n - 1])
+        var left = 0
+        var right = n - 2
+        while left < right {
+            let mid = (left + right) >> 1
+            if presum[mid + 1] >= x {
+                right = mid
+            } else {
+                left = mid + 1
+            }
+        }
+        return left
+    }
+}
+/**
+ * Your Solution object will be instantiated and called as such:
+ * let w = [1]
+ * let solution = Solution(w)
+ * solution.pickIndex()
+ */