feat: add swift implementation to lcci problem: No.08.01

lcci/08.01.Three Steps Problem/README.md

@@ -141,6 +141,26 @@ int waysToStep(int n) {
 }
 ```
 
+```swift
+class Solution {
+    func waysToStep(_ n: Int) -> Int {
+        let mod = Int(1e9) + 7
+        var a = 1, b = 2, c = 4
+        if n == 1 { return a }
+        if n == 2 { return b }
+        if n == 3 { return c }
+
+        for _ in 1..<n {
+            let t = a
+            a = b
+            b = c
+            c = ((a + b) % mod + t) % mod
+        }
+        return a
+    }
+}
+```
+
 <!-- tabs:end -->
 
 ### 方法二：矩阵快速幂加速递推

lcci/08.01.Three Steps Problem/README_EN.md

@@ -143,6 +143,26 @@ int waysToStep(int n) {
 }
 ```
 
+```swift
+class Solution {
+    func waysToStep(_ n: Int) -> Int {
+        let mod = Int(1e9) + 7
+        var a = 1, b = 2, c = 4
+        if n == 1 { return a }
+        if n == 2 { return b }
+        if n == 3 { return c }
+
+        for _ in 1..<n {
+            let t = a
+            a = b
+            b = c
+            c = ((a + b) % mod + t) % mod
+        }
+        return a
+    }
+}
+```
+
 <!-- tabs:end -->
 
 ### Solution 2: Matrix Quick Power to Accelerate Recursion

lcci/08.01.Three Steps Problem/Solution.swift

@@ -0,0 +1,17 @@
+class Solution {
+    func waysToStep(_ n: Int) -> Int {
+        let mod = Int(1e9) + 7
+        var a = 1, b = 2, c = 4
+        if n == 1 { return a }
+        if n == 2 { return b }
+        if n == 3 { return c }
+
+        for _ in 1..<n {
+            let t = a
+            a = b
+            b = c
+            c = ((a + b) % mod + t) % mod
+        }
+        return a
+    }
+}