feat: add swift implementation to lcof2 problem: No.039 (doocs#3049)

Author: klever34

lcof2/剑指 Offer II 039. 直方图最大矩形面积/README.md

@@ -235,6 +235,48 @@ function largestRectangleArea(heights: number[]): number {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func largestRectangleArea(_ heights: [Int]) -> Int {
+        let n = heights.count
+        var left = [Int](repeating: -1, count: n)
+        var right = [Int](repeating: n, count: n)
+        var stack = [Int]()
+
+        for i in 0..<n {
+            while !stack.isEmpty && heights[stack.last!] >= heights[i] {
+                stack.removeLast()
+            }
+            if !stack.isEmpty {
+                left[i] = stack.last!
+            }
+            stack.append(i)
+        }
+
+        stack.removeAll()
+
+        for i in stride(from: n - 1, through: 0, by: -1) {
+            while !stack.isEmpty && heights[stack.last!] >= heights[i] {
+                stack.removeLast()
+            }
+            if !stack.isEmpty {
+                right[i] = stack.last!
+            }
+            stack.append(i)
+        }
+
+        var maxArea = 0
+        for i in 0..<n {
+            maxArea = max(maxArea, (right[i] - left[i] - 1) * heights[i])
+        }
+
+        return maxArea
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 039. 直方图最大矩形面积/Solution.swift

@@ -0,0 +1,37 @@
+class Solution {
+    func largestRectangleArea(_ heights: [Int]) -> Int {
+        let n = heights.count
+        var left = [Int](repeating: -1, count: n)
+        var right = [Int](repeating: n, count: n)
+        var stack = [Int]()
+
+        for i in 0..<n {
+            while !stack.isEmpty && heights[stack.last!] >= heights[i] {
+                stack.removeLast()
+            }
+            if !stack.isEmpty {
+                left[i] = stack.last!
+            }
+            stack.append(i)
+        }
+
+        stack.removeAll()
+
+        for i in stride(from: n - 1, through: 0, by: -1) {
+            while !stack.isEmpty && heights[stack.last!] >= heights[i] {
+                stack.removeLast()
+            }
+            if !stack.isEmpty {
+                right[i] = stack.last!
+            }
+            stack.append(i)
+        }
+
+        var maxArea = 0
+        for i in 0..<n {
+            maxArea = max(maxArea, (right[i] - left[i] - 1) * heights[i])
+        }
+
+        return maxArea
+    }
+}