feat(array): add Dutch National Flag algorithm

array/dutch_national_flag/dutch_national_flag.go

@@ -0,0 +1,27 @@
+package main
+
+import "fmt"
+
+func dutchNationalFlag(arr []int) {
+    low, mid, high := 0, 0, len(arr)-1
+
+    for mid <= high {
+        switch arr[mid] {
+        case 0:
+            arr[low], arr[mid] = arr[mid], arr[low]
+            low++
+            mid++
+        case 1:
+            mid++
+        case 2:
+            arr[high], arr[mid] = arr[mid], arr[high]
+            high--
+        }
+    }
+}
+
+func main() {
+    arr := []int{2, 0, 1, 2, 1, 0}
+    dutchNationalFlag(arr)
+    fmt.Println(arr)  // Should print: [0 0 1 1 2 2]
+}

array/dutch_national_flag/dutch_national_flag.js

@@ -0,0 +1,20 @@
+function dutchNationalFlag(arr) {
+    let low = 0, mid = 0, high = arr.length - 1;
+
+    while (mid <= high) {
+        if (arr[mid] === 0) {
+            [arr[low], arr[mid]] = [arr[mid], arr[low]];
+            low++;
+            mid++;
+        } else if (arr[mid] === 1) {
+            mid++;
+        } else {  // arr[mid] === 2
+            [arr[high], arr[mid]] = [arr[mid], arr[high]];
+            high--;
+        }
+    }
+}
+
+const arr = [2, 0, 1, 2, 1, 0];
+dutchNationalFlag(arr);
+console.log(arr);  // Should print: [0, 0, 1, 1, 2, 2]

array/dutch_national_flag/dutch_national_flag.md

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+Given an array containing only 0s, 1s, and 2s, sort the array in-place in a single pass.
+
+This is also known as the Dutch National Flag problem, named after the Dutch flag's three colors.
+
+Example:
+    Input: [2, 0, 1, 2, 1, 0]
+    Output: [0, 0, 1, 1, 2, 2]
+
+Note:
+    - The array only contains 0s, 1s, and 2s
+    - Must sort in-place with O(n) time complexity and O(1) space complexity

array/dutch_national_flag/dutch_national_flag.py

@@ -0,0 +1,22 @@
+from typing import List
+
+
+def dutch_national_flag(arr: List[int]) -> None:
+    low, mid, high = 0, 0, len(arr) - 1
+
+    while mid <= high:
+        if arr[mid] == 0:
+            arr[low], arr[mid] = arr[mid], arr[low]
+            low += 1
+            mid += 1
+        elif arr[mid] == 1:
+            mid += 1
+        else:  # arr[mid] == 2
+            arr[high], arr[mid] = arr[mid], arr[high]
+            high -= 1
+
+
+if __name__ == "__main__":
+    arr = [2, 0, 1, 2, 1, 0]
+    dutch_national_flag(arr)
+    print(arr)  # Should print: [0, 0, 1, 1, 2, 2]