feat: add swift implementation to lcci problem: No.17.21 (doocs#2808)

Author: klever34

lcci/17.21.Volume of Histogram/README.md

@@ -156,6 +156,38 @@ public class Solution {
 }
 ```
 
+```swift
+class Solution {
+    func trap(_ height: [Int]) -> Int {
+        let n = height.count
+        if n < 3 {
+            return 0
+        }
+
+        var left = [Int](repeating: 0, count: n)
+        var right = [Int](repeating: 0, count: n)
+
+        left[0] = height[0]
+        right[n - 1] = height[n - 1]
+
+        for i in 1..<n {
+            left[i] = max(left[i - 1], height[i])
+        }
+
+        for i in stride(from: n - 2, through: 0, by: -1) {
+            right[i] = max(right[i + 1], height[i])
+        }
+
+        var ans = 0
+        for i in 0..<n {
+            ans += min(left[i], right[i]) - height[i]
+        }
+
+        return ans
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- end -->

lcci/17.21.Volume of Histogram/README_EN.md

@@ -150,6 +150,38 @@ public class Solution {
 }
 ```
 
+```swift
+class Solution {
+    func trap(_ height: [Int]) -> Int {
+        let n = height.count
+        if n < 3 {
+            return 0
+        }
+
+        var left = [Int](repeating: 0, count: n)
+        var right = [Int](repeating: 0, count: n)
+
+        left[0] = height[0]
+        right[n - 1] = height[n - 1]
+
+        for i in 1..<n {
+            left[i] = max(left[i - 1], height[i])
+        }
+
+        for i in stride(from: n - 2, through: 0, by: -1) {
+            right[i] = max(right[i + 1], height[i])
+        }
+
+        var ans = 0
+        for i in 0..<n {
+            ans += min(left[i], right[i]) - height[i]
+        }
+
+        return ans
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- end -->

lcci/17.21.Volume of Histogram/Solution.swift

@@ -0,0 +1,29 @@
+class Solution {
+    func trap(_ height: [Int]) -> Int {
+        let n = height.count
+        if n < 3 {
+            return 0
+        }
+        
+        var left = [Int](repeating: 0, count: n)
+        var right = [Int](repeating: 0, count: n)
+        
+        left[0] = height[0]
+        right[n - 1] = height[n - 1]
+
+        for i in 1..<n {
+            left[i] = max(left[i - 1], height[i])
+        }
+
+        for i in stride(from: n - 2, through: 0, by: -1) {
+            right[i] = max(right[i + 1], height[i])
+        }
+
+        var ans = 0
+        for i in 0..<n {
+            ans += min(left[i], right[i]) - height[i]
+        }
+        
+        return ans
+    }
+}