added Dutch National Flag algorithm fixes [TheAlgorithms#4636]

Author: yatharthmathur

Diff for: sorts/dutch_national_flag.py

@@ -0,0 +1,89 @@
+from typing import List, Any, Tuple
+import random
+
+# These constants stores the relative order of the 3 'colors' 0, 1 and 2.
+# They can be changed as per the requirement of the user.
+# The colors need not be represented by integers, they can also be other comparable or incomparable objects.
+RED = 0 # In this case this is the first color of the flag.
+WHITE = 1 # This is the second color of the flag.
+BLUE = 2 # This is the third color of the flag.
+COLORS = (RED, WHITE, BLUE)
+"""
+The Dutch-National-Flag problem is a problem designed by Edsger Dijkstra. This is called so because the flag of Netherlands
+or Dutch flag is composed of three colors and the sequence of numbers it seeks to sort is composed of 3 unique repeated values.
+These values can be in any order (0,1,2) in our examples but can be made to be anything by assigning the value to RED, WHITE and BLUE
+in that order.
+
+The objective is to sort these n numbers composed of 3 unique repeated values in the most optimal way, that is O(n) (better than the normal
+list.sort() method in Python which has the complexity of O(n)). Another way to solve it is by using Counting Sort but that requires 2
+passes and depending on the size of array could be much more expensive than DNF algorithm (twice as much).
+
+The idea is to maintain 4 regions inside of the sequence provided (marked by l, mid and r index).
+for a sequence of size n:
+Region 1 : sequence[0...l-1] should be colored RED 
+Region 2 : sequence[l...mid-1] should be colored WHITE 
+Region 3 : sequence[mid...r] is an unknown color that we must process
+Region 4 : sequence[r...n] should be colored BLUE
+
+More can be read here : https://en.wikipedia.org/wiki/Dutch_national_flag_problem
+
+"""
+def dutch_national_flag(sequence : List, colors:Tuple[Any]=COLORS) -> None:
+    """
+    This method sorts the sequence of 3 colors given in the order in COLORS constant tuple. This method assumes (0<1<2)
+    but the order can be anything.
+    Inputs : 
+    colors : Tuple[Any] -> a relative ordered tuple of objects that correspond to RED, WHITE and BLUE colors of the flag (in that order).
+    sequence : List[any] -> a sequence of length n with values a[0], a[1], a[2] ... a[n-2], a[n-1] where a[i] in COLORS
+    input example : [0, 0, 2, 2, 2, 1, 1, 2, 2, 1]
+    output example : [0, 0, 1, 1, 1, 2, 2, 2, 2, 2]
+    Sorts the array in place in a single pass. O(n) time complexity and O(1) space complexity
+    >>> from collections import Counter
+    >>> arr1 = [random.choice(COLORS) for _ in range(100)]
+    >>> arr2 = arr1.copy()
+    >>> counter = Counter(arr1)
+    >>> arr1 = [COLORS[0] for _ in range(counter[COLORS[0]])]+[COLORS[1] for _ in range(counter[COLORS[1]])]+\
+[COLORS[2] for _ in range(counter[COLORS[2]])]
+    >>> dutch_national_flag(arr2)
+    >>> arr1 == arr2
+    True
+    """
+    COLORS = colors
+    if sequence is None: return
+    if len(sequence) <= 1: return
+    l = 0
+    mid = 0
+    r = len(sequence)-1
+    while mid <= r:
+        if sequence[mid] == COLORS[0]:
+            sequence[l], sequence[mid] = sequence[mid], sequence[l]
+            l+=1
+            mid+=1
+            continue
+
+        if sequence[mid] == COLORS[1]:
+            mid+=1
+            continue
+
+        if sequence[mid] == COLORS[2]:
+            sequence[r], sequence[mid] = sequence[mid], sequence[r]
+            r-=1
+            continue
+
+        raise ValueError(f"Invalid value {sequence[mid]} inside of sequence. The elements inside the sequence must \
+            be in {COLORS} tuple.")
+
+    return 
+
+
+def main()->None:
+    """
+    Main method to run doctests
+    >>> pass
+    """
+    import doctest
+    doctest.testmod()
+
+
+if __name__ == "__main__":
+    main()