zigzag traversal of a binary tree (#48)

The idea is to use two stacks. We can use one stack for printing from left to right and other stack for printing from right to left. In every iteration, we have nodes of one level in one of the stacks. We print the nodes, and push nodes of next level in other stack.

Author: SakshiiAgrawal

Trees/zigzagtraversal_iterative.py

@@ -0,0 +1,75 @@
+class Node:
+    """
+    A Node has data variable and pointers to its left and right nodes.
+    """
+
+    def __init__(self, data):
+        self.left = None
+        self.right = None
+        self.data = data
+
+def make_tree() -> Node:
+    root = Node(1)
+    root.left = Node(2)
+    root.right = Node(3)
+    root.left.left = Node(4)
+    root.left.right = Node(5)
+    return root
+
+def zigzag_iterative(root: Node):
+    """
+    ZigZag traverse by iterative method: Print node left to right and right to left, alternatively.
+    """
+    if root == None: 
+        return 
+  
+    # two stacks to store alternate levels  
+    s1 = [] # For levels to be printed from right to left  
+    s2 = [] # For levels to be printed from left to right  
+  
+    # append first level to first stack 's1'  
+    s1.append(root)  
+  
+    # Keep printing while any of the stacks has some nodes  
+    while not len(s1) == 0 or not len(s2) == 0: 
+          
+        # Print nodes of current level from s1 and append nodes of next level to s2  
+        while not len(s1) == 0: 
+            temp = s1[-1]  
+            s1.pop()  
+            print(temp.data, end = " ")  
+  
+            # Note that is left is appended before right    
+            if temp.left: 
+                s2.append(temp.left)
+            if temp.right:  
+                s2.append(temp.right)
+  
+        # Print nodes of current level from s2 and append nodes of next level to s1  
+        while not len(s2) == 0: 
+            temp = s2[-1]  
+            s2.pop()  
+            print(temp.data, end = " ")  
+  
+            # Note that is rightt is appended before left   
+            if temp.right:  
+                s1.append(temp.right)
+            if temp.left: 
+                s1.append(temp.left)  
+
+def main():  # Main function for testing.
+    """
+    Create binary tree.
+    """
+    root = make_tree()
+    print("\nZigzag order traversal(iterative) is: ")
+    zigzag_iterative(root)
+    print()
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+    main()
+