feat: add swift implementation to lcof problem: No.43 (doocs#2924)

Author: klever34

lcof/面试题43. 1～n整数中1出现的次数/README.md

@@ -262,6 +262,44 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    private var digits = [Int](repeating: 0, count: 12)
+    private var memo = [[Int?]](repeating: [Int?](repeating: nil, count: 12), count: 12)
+
+    func countDigitOne(_ n: Int) -> Int {
+        var n = n
+        var i = 0
+        while n > 0 {
+            digits[i] = n % 10
+            n /= 10
+            i += 1
+        }
+        return dfs(i - 1, 0, true)
+    }
+
+    private func dfs(_ pos: Int, _ count: Int, _ limit: Bool) -> Int {
+        if pos < 0 {
+            return count
+        }
+        if !limit && memo[pos][count] != nil {
+            return memo[pos][count]!
+        }
+        let upperLimit = limit ? digits[pos] : 9
+        var ans = 0
+        for i in 0...upperLimit {
+            ans += dfs(pos - 1, count + (i == 1 ? 1 : 0), limit && i == upperLimit)
+        }
+        if !limit {
+            memo[pos][count] = ans
+        }
+        return ans
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题43. 1～n整数中1出现的次数/Solution.swift

@@ -0,0 +1,33 @@
+class Solution {
+    private var digits = [Int](repeating: 0, count: 12)
+    private var memo = [[Int?]](repeating: [Int?](repeating: nil, count: 12), count: 12)
+
+    func countDigitOne(_ n: Int) -> Int {
+        var n = n
+        var i = 0
+        while n > 0 {
+            digits[i] = n % 10
+            n /= 10
+            i += 1
+        }
+        return dfs(i - 1, 0, true)
+    }
+
+    private func dfs(_ pos: Int, _ count: Int, _ limit: Bool) -> Int {
+        if pos < 0 {
+            return count
+        }
+        if !limit && memo[pos][count] != nil {
+            return memo[pos][count]!
+        }
+        let upperLimit = limit ? digits[pos] : 9
+        var ans = 0
+        for i in 0...upperLimit {
+            ans += dfs(pos - 1, count + (i == 1 ? 1 : 0), limit && i == upperLimit)
+        }
+        if !limit {
+            memo[pos][count] = ans
+        }
+        return ans
+    }
+}