chore(Python): add dutch national flag algo (#720)

algorithms/Python/README.md

@@ -6,6 +6,7 @@
 - [Rotate Array](arrays/rotate_array.py)
 - [Missing Number](arrays/missing_number.py)
 - [Remove duplicate items](arrays/remove_duplicates_list.py)
+- [Dutch National Flag Algorithm](arrays/dutch_national_flag_algo.py)
 
 ## Linked Lists
 - [Doubly](linked_lists/doubly.py)

algorithms/Python/arrays/dutch_national_flag_algo.py

@@ -0,0 +1,39 @@
+"""
+Algorithm Type: Array sorting with 0s,1s and 2s in a single pass
+Explain: To separate three different groups
+Time Complexity: O(n)
+Space Complexity: O(1)
+
+    0 0 1 1 1 ? ? ? ? 2 2
+        |     |     |
+        v     v     v
+       Low   Mid   High
+
+   To goal of the algo is to shrink this '?' region
+   wrapped by Mid and High
+
+   > Low starts at 1
+"""
+
+numbers = [2, 0, 1, 1, 2, 0, 1, 2]
+
+
+def DNFS(numbers: list) -> list:
+    length = len(numbers)
+    low = 0
+    high = length - 1
+    mid = 0
+    while mid <= high:
+        if numbers[mid] == 0:
+            numbers[low], numbers[mid] = numbers[mid], numbers[low]
+            low = low + 1
+            mid = mid + 1
+        elif numbers[mid] == 1:
+            mid = mid + 1
+        else:
+            numbers[mid], numbers[high] = numbers[high], numbers[mid]
+            high = high - 1
+    return numbers
+
+if __name__ == "__main__":
+    print(DNFS(numbers))