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Reworked notebook: Added more detail to constraints and test cases. Reworked algorithm discussion and unit test.

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Donne Martin 2015-06-26 05:06:11 -04:00
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143
linked-lists/find-loop-start.ipynb
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@ -13,22 +13,27 @@
"source": [
"## Problem: Find the start of a linked list loop.\n",
"\n",
"* [Clarifying Questions](#Clarifying-Questions)\n",
"* [Constraints and Assumptions](#Constraints-and-Assumptions)\n",
"* [Test Cases](#Test-Cases)\n",
"* [Algorithm](#Algorithm)\n",
"* [Code](#Code)"
"* [Code](#Code)\n",
"* [Unit Test](#Unit-Test)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Clarifying Questions\n",
"## Constraints and Assumptions\n",
"\n",
"*Problem statements are often intentionally ambiguous. Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n",
"\n",
"* This is a singly linked list?\n",
" * Yes\n",
"* Can we assume we are always passed a circular linked list?\n",
" * No"
" * No\n",
"* Can we assume we already have a linked list class that can be used for this problem?\n",
" * Yes"
]
},
{
@ -51,10 +56,10 @@
"source": [
"## Algorithm\n",
"\n",
"* Check for an empty list\n",
"* Use two pointers i, j, initialized to the head\n",
"* j is incremented twice as fast as i\n",
" * If j's next is NULL, we do not have a circular list\n",
"* Increment i and j until they meet\n",
" * j is incremented twice as fast as i\n",
" * If j's next is NULL, we do not have a circular list\n",
"* When i and j meet, move j to the head\n",
"* Increment i and j one node at a time until they meet\n",
"* Where they meet is the start of the loop\n",
@ -73,7 +78,7 @@
},
{
"cell_type": "code",
"execution_count": null,
"execution_count": 1,
"metadata": {
"collapsed": true
},
@ -84,7 +89,7 @@
},
{
"cell_type": "code",
"execution_count": null,
"execution_count": 2,
"metadata": {
"collapsed": false
},
@ -98,65 +103,103 @@
" j = self.head\n",
" i = i.next\n",
" j = j.next.next\n",
" \n",
" # Increment i and j until they meet\n",
" # j is incremented twice as fast as i\n",
" while j != i:\n",
" i = i.next\n",
" if j is None or j.next is None:\n",
" return\n",
" j = j.next.next\n",
" \n",
" # When i and j meet, move j to the head\n",
" j = self.head\n",
" \n",
" # Increment i and j one node at a time until \n",
" # they meet, which is the start of the loop\n",
" while j != i:\n",
" i = i.next\n",
" j = j.next\n",
" return i.data"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Unit Test"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"*It is important to identify and run through general and edge cases from the [Test Cases](#Test-Cases) section by hand. You generally will not be asked to write a unit test like what is shown below.*"
]
},
{
"cell_type": "code",
"execution_count": null,
"execution_count": 3,
"metadata": {
"collapsed": false
},
"outputs": [],
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Test: Empty list\n",
"Test: Not a circular linked list: One element\n",
"Test: Not a circular linked list: Two elements\n",
"Test: Not a circular linked list: Three or more elements\n",
"Test: General case: Circular linked list\n",
"Success: test_find_loop_start\n"
]
}
],
"source": [
"print('Empty list')\n",
"linked_list = MyLinkedList()\n",
"print(linked_list.find_loop_start())\n",
"print('Not a circular linked list')\n",
"print('One element')\n",
"head = Node(1)\n",
"linked_list = MyLinkedList(head)\n",
"print(linked_list.find_loop_start())\n",
"print('Two elements')\n",
"linked_list.append(2)\n",
"print(linked_list.find_loop_start())\n",
"print('Three or more elements')\n",
"linked_list.append(3)\n",
"print(linked_list.find_loop_start())\n",
"print('General case')\n",
"node0 = Node(0)\n",
"node1 = Node(1)\n",
"node2 = Node(2)\n",
"node3 = Node(3)\n",
"node4 = Node(4)\n",
"node5 = Node(5)\n",
"node6 = Node(6)\n",
"node7 = Node(7)\n",
"node8 = Node(8)\n",
"node9 = Node(9)\n",
"node10 = Node(10)\n",
"node0.next = node1\n",
"node1.next = node2\n",
"node2.next = node3\n",
"node3.next = node4\n",
"node4.next = node5\n",
"node5.next = node6\n",
"node6.next = node7\n",
"node7.next = node8\n",
"node8.next = node9\n",
"node9.next = node10\n",
"node10.next = node3\n",
"linked_list = MyLinkedList(node0)\n",
"print(linked_list.find_loop_start())"
"from nose.tools import assert_equal\n",
"\n",
"class Test(object):\n",
" def test_find_loop_start(self):\n",
" print('Test: Empty list')\n",
" linked_list = MyLinkedList()\n",
" assert_equal(linked_list.find_loop_start(), None)\n",
" \n",
" print('Test: Not a circular linked list: One element')\n",
" head = Node(1)\n",
" linked_list = MyLinkedList(head)\n",
" assert_equal(linked_list.find_loop_start(), None)\n",
" \n",
" print('Test: Not a circular linked list: Two elements')\n",
" linked_list.append(2)\n",
" assert_equal(linked_list.find_loop_start(), None)\n",
" \n",
" print('Test: Not a circular linked list: Three or more elements')\n",
" linked_list.append(3)\n",
" assert_equal(linked_list.find_loop_start(), None)\n",
" \n",
" print('Test: General case: Circular linked list')\n",
" node10 = Node(10)\n",
" node9 = Node(9, node10)\n",
" node8 = Node(8, node9)\n",
" node7 = Node(7, node8)\n",
" node6 = Node(6, node7)\n",
" node5 = Node(5, node6)\n",
" node4 = Node(4, node5)\n",
" node3 = Node(3, node4)\n",
" node2 = Node(2, node3)\n",
" node1 = Node(1, node2)\n",
" node0 = Node(0, node1)\n",
" node10.next = node3\n",
" linked_list = MyLinkedList(node0)\n",
" assert_equal(linked_list.find_loop_start(), 3)\n",
" \n",
" print('Success: test_find_loop_start')\n",
"\n",
"if __name__ == '__main__':\n",
" test = Test()\n",
" test.test_find_loop_start()"
]
}
],