Add string palindrome algorithm

algorithms/palindrome/palindrome_indices.swift

@@ -0,0 +1,32 @@
+// A palindrome is a string that reads the same forwards and backwards.
+//
+// Examples: "level", "radar", "madam", "A man, a plan, a canal: Panama".
+
+extension String {
+
+    /// 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() -> Bool {
+        var leftIndex = startIndex
+        var rightIndex = index(before: endIndex)
+
+        while leftIndex < rightIndex {
+            guard self[leftIndex].isLetter || self[leftIndex].isNumber else {
+                leftIndex = index(after: leftIndex)
+                continue
+            }
+            guard self[rightIndex].isLetter || self[rightIndex].isNumber else {
+                rightIndex = index(before: rightIndex)
+                continue
+            }
+            guard self[leftIndex].lowercased() == self[rightIndex].lowercased() else {
+                return false
+            }
+
+            leftIndex = index(after: leftIndex)
+            rightIndex = index(before: rightIndex)
+        }
+
+        return true
+    }
+}

algorithms/palindrome/palindrome_recursion.swift

@@ -0,0 +1,44 @@
+// A palindrome is a string that reads the same forwards and backwards.
+//
+// Examples: "level", "radar", "madam", "A man, a plan, a canal: Panama".
+
+extension String {
+
+    /// 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() -> Bool {
+        isPalindromeRecursion(
+            leftIndex: startIndex,
+            rightIndex: index(before: endIndex)
+        )
+    }
+
+    private func isPalindromeRecursion(
+        leftIndex: String.Index,
+        rightIndex: String.Index
+    ) -> Bool {
+        guard leftIndex < rightIndex else {
+            return true
+        }
+        guard self[leftIndex].isLetter || self[leftIndex].isNumber else {
+            return isPalindromeRecursion(
+                leftIndex: index(after: leftIndex),
+                rightIndex: rightIndex
+            )
+        }
+        guard self[rightIndex].isLetter || self[rightIndex].isNumber else {
+            return isPalindromeRecursion(
+                leftIndex: leftIndex,
+                rightIndex: index(before: rightIndex)
+            )
+        }
+        guard self[leftIndex].lowercased() == self[rightIndex].lowercased() else {
+            return false
+        }
+
+        return isPalindromeRecursion(
+            leftIndex: index(after: leftIndex),
+            rightIndex: index(before: rightIndex)
+        )
+    }
+}

algorithms/palindrome/palindrome_reversed.swift

@@ -0,0 +1,13 @@
+// A palindrome is a string that reads the same forwards and backwards.
+//
+// Examples: "level", "radar", "madam", "A man, a plan, a canal: Panama".
+
+extension String {
+
+    /// 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() -> Bool {
+        let input = lowercased().filter { $0.isLetter || $0.isNumber }
+        return input == String(input.reversed())
+    }
+}