Added Node in search.ipynb (#761)

Author: SkAdilina

search.ipynb

@@ -15,6 +15,7 @@
    "cell_type": "code",
    "execution_count": 1,
    "metadata": {
+    "collapsed": true,
     "scrolled": true
    },
    "outputs": [],
@@ -82,7 +83,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 2,
    "metadata": {
     "collapsed": true
    },
@@ -115,6 +116,51 @@
     "* `value(self, state)` : This acts as a bit of extra information in problems where we try to optimise a value when we cannot do a goal test."
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## NODE\n",
+    "\n",
+    "Let's see how we define a Node. Run the next cell to see how abstract class `Node` is defined in the search module."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 7,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "%psource Node"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "The `Node` class has nine methods.\n",
+    "\n",
+    "* `__init__(self, state, parent, action, path_cost)` : This method creates a node. `parent` represents the the node that this is a successor of and `action` is the action required to get from the parent node to this node. `path_cost` is the cost to reach current node from parent node.\n",
+    "\n",
+    "* `__repr__(self)` : This returns the state of this node.\n",
+    "\n",
+    "* `__lt__(self, node)` : Given a `node`, this method returns `True` if the state of current node is less than the state of the `node`. Otherwise it returns `False`.\n",
+    "\n",
+    "* `expand(self, problem)` : This methods lists all the neighbouring(reachable in one step) nodes of current node. \n",
+    "\n",
+    "* `child_node(self, problem, action)` : Given an `action`, this methods returns the immediate neighbour that can be reached with that `action`.\n",
+    "\n",
+    "* `solution(self)` : This returns the sequence of actions required to reach this node from the root node. \n",
+    "\n",
+    "* `path(self)` : This returns a list of all the nodes that lies in the path from the root to this node.\n",
+    "\n",
+    "* `__eq__(self, other)` : This method returns `True` if the state of current node is equal to the other node. Else it returns `False`.\n",
+    "\n",
+    "* `__hash__(self)` : This returns the hash of the state of current node."
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},