feat: add longest alternating subsequence

docs/source/pydatastructs/linear_data_structures/algorithms.rst

@@ -45,4 +45,6 @@ Algorithms
 
 .. autofunction:: pydatastructs.jump_search
 
-.. autofunction:: pydatastructs.intro_sort
+.. autofunction:: pydatastructs.intro_sort
+
+.. autofunction:: pydatastructs.longest_increasing_subsequence

pydatastructs/linear_data_structures/__init__.py

@@ -47,6 +47,7 @@
     jump_search,
     selection_sort,
     insertion_sort,
-    intro_sort
+    intro_sort,
+    longest_alternating_subsequence,
 )
 __all__.extend(algorithms.__all__)

pydatastructs/linear_data_structures/algorithms.py

@@ -30,7 +30,8 @@
     'jump_search',
     'selection_sort',
     'insertion_sort',
-    'intro_sort'
+    'intro_sort',
+    'longest_alternating_subsequence',
 ]
 
 def _merge(array, sl, el, sr, er, end, comp):
@@ -1850,3 +1851,69 @@ def partition(array, lower, upper):
         intro_sort(array, start=p+1, end=upper,  maxdepth=maxdepth-1, ins_threshold=ins_threshold)
 
         return array
+
+def longest_alternating_subsequence(array: OneDimensionalArray, **kwargs) -> int:
+    """
+    Finds the longest alternating subsequence in the given array.
+    An alternating sequence is one in which the elements are in alternating order,
+    i.e., a sequence where the differences between consecutive elements alternate
+    between positive and negative.
+
+    Parameters
+    ==========
+
+    array: OneDimensionalArray
+        The array from which the longest alternating subsequence is to be found.
+    backend: pydatastructs.Backend
+        The backend to be used.
+        Optional, by default, the best available backend is used.
+
+    Returns
+    =======
+
+    output: OneDimensionalArray
+        The longest alternating subsequence.
+
+    Examples
+    ========
+
+    >>> from pydatastructs import OneDimensionalArray, longest_alternating_subsequence
+    >>> arr = OneDimensionalArray(int, [1, 5, 4])
+    >>> las = longest_alternating_subsequence(arr)
+    >>> las
+    3
+    >>> arr = OneDimensionalArray(int, [1, 5, 4, 3, 2, 6, 7])
+    >>> las = longest_alternating_subsequence(arr)
+    >>> las
+    4
+
+    References
+    ==========
+
+    .. [1] https://en.wikipedia.org/wiki/Longest_alternating_subsequence
+
+    Note
+    ====
+
+    The data types of elements in the array should be comparable.
+    """
+    raise_if_backend_is_not_python(
+        longest_alternating_subsequence, kwargs.get('backend', Backend.PYTHON))
+
+    increasing = 1
+    decreasing = 1
+    n = len(array)
+
+    # Iterate from second element
+    for i in range(1, n):
+
+        # Increasing changes if decreasing changes
+        if (array[i] > array[i-1]):
+            increasing = decreasing + 1
+
+        # Decreasing changes if increasing changes
+        elif (array[i] < array[i-1]):
+            decreasing = increasing + 1
+
+    # Return the maximum length
+    return max(increasing, decreasing)

pydatastructs/linear_data_structures/tests/test_algorithms.py

@@ -5,7 +5,7 @@
     cocktail_shaker_sort, quick_sort, longest_common_subsequence, is_ordered,
     upper_bound, lower_bound, longest_increasing_subsequence, next_permutation,
     prev_permutation, bubble_sort, linear_search, binary_search, jump_search,
-    selection_sort, insertion_sort, intro_sort, Backend)
+    selection_sort, insertion_sort, intro_sort, longest_alternating_subsequence, Backend)
 
 from pydatastructs.utils.raises_util import raises
 import random
@@ -414,3 +414,32 @@ def test_binary_search():
 def test_jump_search():
     _test_common_search(jump_search)
     _test_common_search(jump_search, backend=Backend.CPP)
+
+
+def test_longest_alternating_subsequence():
+    ODA = OneDimensionalArray
+
+    arr1 = ODA(int, [-4, 3, -5, 9, 10, 12, 2, -1])
+    output: int = longest_alternating_subsequence(arr1)
+    expected_result = [3, -5, 9, 2, -1]
+    assert len(expected_result) == output
+
+    arr2 = ODA(int, [10, 22, 9, 33, 49, 50, 31, 60])
+    output: int = longest_alternating_subsequence(arr2)
+    expected_result = [10, 22, 9, 33, 31, 60]
+    assert len(expected_result) == output
+
+    arr3 = ODA(int, [1, 2, 3, 4, 5, 6, 7, 8, 9])
+    output: int = longest_alternating_subsequence(arr3)
+    expected_result = [1, 2]
+    assert len(expected_result) == output
+
+    arr4 = ODA(int, [9, 8, 7, 6, 5, 4, 3, 2, 1])
+    output: int = longest_alternating_subsequence(arr4)
+    expected_result = [9, 8]
+    assert len(expected_result) == output
+
+    arr5 = ODA(int, [1, 5, 4, 3, 2, 1, 6, 7, 8, 9])
+    output: int = longest_alternating_subsequence(arr5)
+    expected_result = [1, 5, 4, 6]
+    assert len(expected_result) == output