Adding a program for swap nodes in linkedlist (#667)

* Adding a program for swap nodes in linkedlist

* Updating swapNodes

data_structures/linked_list/swapNodes.py

@@ -0,0 +1,75 @@
+class Node:
+    def __init__(self, data):
+        self.data = data;
+        self.next = None
+
+
+class Linkedlist:
+    def __init__(self):
+        self.head = None
+
+    def print_list(self):
+        temp = self.head
+        while temp is not None:
+            print(temp.data)
+            temp = temp.next
+
+# adding nodes
+    def push(self, new_data):
+        new_node = Node(new_data)
+        new_node.next = self.head
+        self.head = new_node
+
+# swapping nodes
+    def swapNodes(self, d1, d2):
+        prevD1 = None
+        prevD2 = None
+        if d1 == d2:
+            return
+        else:
+            # find d1
+            D1 = self.head
+            while D1 is not None and D1.data != d1:
+                prevD1 = D1
+                D1 = D1.next
+                # find d2
+            D2 = self.head
+            while D2 is not None and D2.data != d2:
+                prevD2 = D2
+                D2 = D2.next
+            if D1 is None and D2 is None:
+                return
+            # if D1 is head
+            if prevD1 is not None:
+                prevD1.next = D2
+            else:
+                self.head = D2
+            # if D2 is head
+            if prevD2 is not None:
+                prevD2.next = D1
+            else:
+                self.head = D1
+            temp = D1.next
+            D1.next = D2.next
+            D2.next = temp
+
+# swapping code ends here
+
+
+
+if __name__ == '__main__':
+    list = Linkedlist()
+    list.push(5)
+    list.push(4)
+    list.push(3)
+    list.push(2)
+    list.push(1)
+
+    list.print_list()
+
+    list.swapNodes(1, 4)
+    print("After swapping")
+    list.print_list()
+
+
+