feat: Add DutchNationalFlag algorithm #40

Sources/TwoPointer/DutchNationalFlag.swift

@@ -0,0 +1,62 @@
+//
+//  File.swift
+//  SwiftyAlgorithms
+//
+//  Created by Mohshinsha Shahmadar on 2025-04-09.
+//
+
+import Foundation
+
+/**
+ Given an array nums with n objects coloured red, white, or blue, sort them In-place and one-pass (https://en.wikipedia.org/wiki/In-place_algorithm) so that objects of the same color are adjacent, with the colors in the order red, white, and blue.
+
+ We will use the integers 0, 1, and 2 to represent the color red, white, and blue, respectively.
+
+ You must solve this problem without using the library's sort function.
+
+ Example 1:
+
+ Input: nums = [2,0,2,1,1,0]
+ Output: [0,0,1,1,2,2]
+ Example 2:
+
+ Input: nums = [2,0,1]
+ Output: [0,1,2]
+ Constraints:
+
+ n == nums.length
+ 1 <= n <= 300
+ nums[i] is either 0, 1, or 2.
+ */
+
+func dutchNationalFlag(_ nums: inout [Int]) {
+
+  var k = nums.endIndex - 1
+  var i = 0
+  var j = 0
+
+  // Define regions
+  // 0 to j-1 => 0s
+  // j to i-1 => 1s
+  // i to k -> Unknown
+  // k to end -> 2s
+
+  while i <= k {
+    switch nums[i] {
+
+    case 0:
+      nums.swapAt(i, j)
+      j += 1
+      i += 1
+
+    case 1:
+      i += 1
+
+    case 2:
+      nums.swapAt(i, k)
+      k -= 1
+
+    default: break
+    }
+  }
+}

Tests/SwiftyAlgorithmsTests/TwoPointer/DutchNationalFlagTests.swift

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+//
+//  DutchNationalFlagTests.swift
+//  SwiftyAlgorithms
+//
+//  Created by Mohshinsha Shahmadar on 2025-04-09.
+//
+
+import XCTest
+@testable import SwiftyAlgorithms
+
+final class DutchNationalFlagTests: XCTestCase {
+
+  func testDutchNationalFlag() async throws {
+    let inputs = [[2, 0, 2, 1, 1, 0], [2, 0, 1]]
+    let expectedOutputs = [[0, 0, 1, 1, 2, 2], [0, 1, 2]]
+
+    for (index, inputArray) in inputs.enumerated() {
+      var input = inputArray
+      dutchNationalFlag(&input)
+      XCTAssertEqual(input, expectedOutputs[index], "Failed at index \(index) with input \(inputArray)")
+    }
+  }
+}