Add string palindrome algorithm

Author: nktssk

algorithms/palindrome/palindrome.swift

@@ -0,0 +1,88 @@
+// A palindrome is a string that reads the same forwards and backwards.
+//
+// Examples: "level", "radar", "madam", "A man, a plan, a canal: Panama".
+
+struct PalindromeSolutionFirst {
+    
+    /// Using the `reverse()` method to reverse the string and comparing it with the original. Only include letters and numbers.
+    /// - Complexity: O(n), with allocating O(n) space.
+    func isPalindrome(_ input: String) -> Bool {
+        let input = input.lowercased().filter { $0.isLetter || $0.isNumber }
+        return input == String(input.reversed())
+    }
+}
+
+struct PalindromeSolutionSecond {
+    
+    /// Iteratively comparing characters from the beginning and end of the string. Only include letters and numbers.
+    /// - Complexity: O(n), without allocating new space.
+    func isPalindrome(_ input: String) -> Bool {
+        var leftIndex = input.startIndex
+        var rightIndex = input.index(before: input.endIndex)
+        
+        while leftIndex < rightIndex {
+            if !input[leftIndex].isLetter && !input[leftIndex].isNumber {
+                leftIndex = input.index(after: leftIndex)
+                continue
+            }
+            if !input[rightIndex].isLetter && !input[rightIndex].isNumber {
+                rightIndex = input.index(before: rightIndex)
+                continue
+            }
+            if input[leftIndex].lowercased() != input[rightIndex].lowercased() {
+                return false
+            }
+            
+            leftIndex = input.index(after: leftIndex)
+            rightIndex = input.index(before: rightIndex)
+        }
+        
+        return true
+    }
+}
+
+struct PalindromeSolutionThird {
+    
+    /// Recursively comparing characters from the beginning and end of the string. Only include letters and numbers.
+    /// - Complexity: O(n), without allocating new space.
+    func isPalindrome(_ input: String) -> Bool {
+        isPalindromeRecursion(
+            input,
+            leftIndex: input.startIndex,
+            rightIndex: input.index(before: input.endIndex)
+        )
+    }
+    
+    private func isPalindromeRecursion(
+        _ input: String,
+        leftIndex: String.Index,
+        rightIndex: String.Index
+    ) -> Bool {
+        if leftIndex >= rightIndex {
+            return true
+        }
+        if !input[leftIndex].isLetter && !input[leftIndex].isNumber {
+            return isPalindromeRecursion(
+                input,
+                leftIndex: input.index(after: leftIndex),
+                rightIndex: rightIndex
+            )
+        }
+        if !input[rightIndex].isLetter && !input[rightIndex].isNumber {
+            return isPalindromeRecursion(
+                input,
+                leftIndex: leftIndex,
+                rightIndex: input.index(before: rightIndex)
+            )
+        }
+        if input[leftIndex].lowercased() != input[rightIndex].lowercased() {
+            return false
+        }
+        
+        return isPalindromeRecursion(
+            input,
+            leftIndex: input.index(after: leftIndex),
+            rightIndex: input.index(before: rightIndex)
+        )
+    }
+}