Merge pull request #42 from miguelHx/miguel_2.6_palindrome

Miguel 2.6 - Palindrome [Python]

Author: lhchavez

Python/chapter02/2.6 - Palindrome/miguel_2.6_soln.py

@@ -0,0 +1,267 @@
+"""
+Python version 3.7.0
+2.6 - Palindrome
+Implement a function to check if a linked list
+is a palindrome.
+
+"""
+import unittest
+
+
+class Node:
+    def __init__(self, d: int):
+        self.data = d
+        self.next = None
+
+    def __repr__(self):
+        return self.__str__()
+
+    def __str__(self):
+        return '<Node Value: {}>'.format(self.data)
+
+    def __eq__(self, other: object):
+        if not isinstance(other, Node):
+            return NotImplemented
+        return self.data == other.data
+
+
+class LinkedList:
+    def __init__(self, *numbers: int):
+        self.head = None
+        self.tail = None
+        self.size = 0
+        for num in numbers:
+            self.append_to_tail(num)
+
+    def append_to_tail(self, e) -> None:
+        if isinstance(e, int):
+            self._append_num(e)
+        elif isinstance(e, Node):
+            self._append_node(e)
+
+    def _append_num(self, d: int) -> None:
+        if self.head is None:
+            self.head = Node(d)
+            self.tail = self.head
+        else:
+            end = Node(d)
+            self.tail.next = end
+            self.tail = end
+        self.size += 1
+
+    def _append_node(self, n: Node) -> None:
+        if self.head is None:
+            self.head = n
+            self.tail = self.head
+        else:
+            end = n
+            self.tail.next = end
+            self.tail = end
+        self.size += 1
+
+    def append_to_head(self, d: int) -> None:
+        new_head = Node(d)
+        new_head.next = self.head
+        if self.head is None:
+            # if list is empty and we add
+            # out first element, head AND tail
+            # must point to same node
+            self.tail = new_head
+        self.head = new_head
+        self.size += 1
+
+    def get_node_at(self, index: int) -> Node:
+        if index < 0 or index >= self.size:
+            raise IndexError('list index out of range')
+        n = self.head
+        for i in range(self.size):
+            if i == index:
+                return n
+            n = n.next
+
+    def get_value_at(self, index: int) -> int:
+        if index < 0 or index >= self.size:
+            raise IndexError('list index out of range')
+        n = self.head
+        for i in range(self.size):
+            if i == index:
+                return n.data
+            n = n.next
+
+    def pop_head(self) -> Node:
+        if self.head is None:
+            raise IndexError('no head to pop')
+        h = self.head
+        h.next = None
+        self.head = self.head.next
+        self.size -= 1
+        return h
+
+    def append(self, ll: 'LinkedList') -> None:
+        self.tail.next = ll.head
+        self.size += ll.size
+        ll.head = None
+        ll.size = 0
+
+    def reverse(self) -> None:
+        """
+        Reverses this linked list in place
+        :return:
+        """
+        if self.head is None:
+            return
+        prev = self.head
+        self.tail = prev
+        curr = prev.next
+        self.tail.next = None
+        while curr is not None:
+            old_next = curr.next
+            curr.next = prev
+            prev = curr
+            curr = old_next
+        self.head = prev
+
+    def __repr__(self):
+        return self.__str__()
+
+    def __str__(self):
+        if self.head is None:
+            return '<empty>'
+        ll = []
+        n = self.head
+        while n.next is not None:
+            ll.append('{} -> '.format(n.data))
+            n = n.next
+        ll.append(str(n.data))
+        return ''.join(ll)
+
+    def __eq__(self, other: object):
+        if not isinstance(other, LinkedList):
+            return NotImplemented
+        a = self.head
+        b = other.head
+        while a is not None and b is not None:
+            if a.data != b.data:
+                return False
+            # otherwise, advance both pointers
+            a = a.next
+            b = b.next
+        return a is None and b is None
+
+
+def reverse_linked_list(ll: LinkedList) -> LinkedList:
+    """
+    Takes in a linked list and returns a reversed copy
+    :param ll: input linked list
+    :return: reversed linked list
+    """
+    output_ll = LinkedList()
+    n = ll.head
+    while n is not None:
+        output_ll.append_to_head(n.data)
+        n = n.next
+    return output_ll
+
+
+def is_palindrome_constant_space(ll: LinkedList) -> bool:
+    """
+    Same as the palindrome function below, but
+    with constant space
+    Runtime:  O(n)
+    Space Complexity:  O(1)
+    :param ll: an input linked list
+    :return: true if ll is a palindrome, false otherwise
+    """
+    # reverse half of the linked list
+    temp_ll1 = LinkedList()
+    temp_ll2 = LinkedList()
+    n = ll.head
+    for _ in range((ll.size + 1) // 2):
+        temp_ll1.append_to_tail(n)  # first half
+        n = n.next
+    while n is not None:
+        temp_ll2.append_to_tail(n)  # second half
+        n = n.next
+    # reverse second half
+    temp_ll2.reverse()
+    # compare temp_ll1 and temp_ll2
+    n1 = temp_ll1.head
+    n2 = temp_ll2.head
+    for _ in range(temp_ll2.size):
+        if n1.data != n2.data:
+            temp_ll2.reverse()
+            return False
+        n1 = n1.next
+        n2 = n2.next
+    temp_ll2.reverse()
+    return True
+
+
+def is_palindrome(ll: LinkedList) -> bool:
+    """
+    Given a linked list, this function will check if the
+    linked list is a palindrome.
+    A palindrome is a word or phrase that is the same
+    forwards and backwards.
+    Since I will use integer nodes for the linked list,
+    we will be checking if the sequence of numbers
+    in a linked list is a palindrome.
+    Runtime:  O(n)
+    Space Complexity:  O(n)
+    :param ll: an input linked list
+    :return: true if ll is a palindrome, false otherwise
+    """
+    return ll == reverse_linked_list(ll)
+
+
+class TestIsPalindrome(unittest.TestCase):
+
+    def setUp(self):
+        self.test_cases = [
+            (
+                LinkedList(1, 2, 3, 4, 3, 2, 1),
+                True
+            ),
+            (
+                LinkedList(1, 2, 3, 2, 1),
+                True
+            ),
+            (
+                LinkedList(1, 2, 3, 4, 2, 1),
+                False
+            ),
+            (
+                LinkedList(4, 5, 6, 7, 8, 9),
+                False
+            ),
+            (
+                LinkedList(99, 0),
+                False
+            ),
+            (
+                LinkedList(100, 100),
+                True
+            ),
+            (
+                LinkedList(7),
+                True
+            ),
+            (
+                LinkedList(),
+                True
+            )
+        ]
+
+    def test_is_palindrome(self):
+        for ll, expected in self.test_cases:
+            self.assertEqual(is_palindrome(ll), expected, msg=ll)
+
+    def test_is_palindrome_constant_space(self):
+        for ll, expected in self.test_cases:
+            ll_temp = LinkedList(*[ll.get_value_at(i) for i in range(ll.size)])
+            self.assertEqual(is_palindrome_constant_space(ll), expected, msg=ll)
+            self.assertEqual(ll_temp, ll)
+
+
+if __name__ == '__main__':
+    unittest.main()