Add Permutation

data_structures/arrays/Permutations.swift

@@ -0,0 +1,53 @@
+//
+//  Permutations.swift
+//  
+//
+//  Created by Moon Jongseek on 11/22/24.
+//
+
+/// Returns the array of permutation with the specified length.
+///
+/// Generates permutations in ascending order based on the index.
+/// The following example shows how to generate permutaions for the `Int` type:
+///
+///     let permutationSet = permutations(data: [1,2,3,4], count: 2)
+///     // [[1, 2], [1, 3], [1, 4], [2, 1], [2, 3], [2, 4], [3, 1], [3, 2], [3, 4], [4, 1], [4, 2], [4, 3]]
+///
+/// - Parameters:
+///   - data: The array data to generate permutations.
+///   - count: The number of each permutation elements.
+///
+/// - Returns: An array of permutations with length `count`. if `count` is greater than the length of `data` or less than 0, returns `nil`.
+///
+/// - Complexity: O(n! * n)
+///
+func permutations<T>(data: [T], count: Int) -> [[T]]? {
+    guard data.count >= count && count >= 0 else { return nil }
+    var result: [[T]] = []
+    var buffer: [T] = []
+    var visited = Array(repeating: false, count: data.count)
+
+    func nextElement() {
+        if buffer.count == count {
+            result.append(buffer)
+            return
+        }
+        for i in 0..<data.count {
+            if !visited[i] {
+                visited[i] = true
+                buffer.append(data[i])
+                nextElement()
+                buffer.removeLast()
+                visited[i] = false
+            }
+        }
+    }
+    nextElement()
+    return result
+}
+
+let permutation_test0 = permutations(data: [1,2,3,4], count: 2)
+print(permutation_test0)
+
+let permutation_test1 = permutations(data: [1,2,3,4], count: -1)
+print(permutation_test1)