feat: add swift implementation to lcof2 problem: No.040 (doocs#3050)

Author: klever34

lcof2/剑指 Offer II 040. 矩阵中最大的矩形/README.md

@@ -254,6 +254,57 @@ func largestRectangleArea(heights []int) int {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func maximalRectangle(_ matrix: [String]) -> Int {
+        guard let firstRow = matrix.first else {
+            return 0
+        }
+
+        let n = firstRow.count
+        var heights = [Int](repeating: 0, count: n)
+        var ans = 0
+
+        for row in matrix {
+            for (j, char) in row.enumerated() {
+                if char == "1" {
+                    heights[j] += 1
+                } else {
+                    heights[j] = 0
+                }
+            }
+            ans = max(ans, largestRectangleArea(heights))
+        }
+
+        return ans
+    }
+
+    private func largestRectangleArea(_ heights: [Int]) -> Int {
+        var res = 0
+        let n = heights.count
+        var stack = [Int]()
+        var left = [Int](repeating: -1, count: n)
+        var right = [Int](repeating: n, count: n)
+
+        for i in 0..<n {
+            while !stack.isEmpty && heights[stack.last!] >= heights[i] {
+                right[stack.removeLast()] = i
+            }
+            left[i] = stack.isEmpty ? -1 : stack.last!
+            stack.append(i)
+        }
+
+        for i in 0..<n {
+            res = max(res, heights[i] * (right[i] - left[i] - 1))
+        }
+
+        return res
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 040. 矩阵中最大的矩形/Solution.swift

@@ -0,0 +1,46 @@
+class Solution {
+    func maximalRectangle(_ matrix: [String]) -> Int {
+        guard let firstRow = matrix.first else {
+            return 0
+        }
+
+        let n = firstRow.count
+        var heights = [Int](repeating: 0, count: n)
+        var ans = 0
+
+        for row in matrix {
+            for (j, char) in row.enumerated() {
+                if char == "1" {
+                    heights[j] += 1
+                } else {
+                    heights[j] = 0
+                }
+            }
+            ans = max(ans, largestRectangleArea(heights))
+        }
+
+        return ans
+    }
+
+    private func largestRectangleArea(_ heights: [Int]) -> Int {
+        var res = 0
+        let n = heights.count
+        var stack = [Int]()
+        var left = [Int](repeating: -1, count: n)
+        var right = [Int](repeating: n, count: n)
+
+        for i in 0..<n {
+            while !stack.isEmpty && heights[stack.last!] >= heights[i] {
+                right[stack.removeLast()] = i
+            }
+            left[i] = stack.isEmpty ? -1 : stack.last!
+            stack.append(i)
+        }
+
+        for i in 0..<n {
+            res = max(res, heights[i] * (right[i] - left[i] - 1))
+        }
+
+        return res
+    }
+}