# Tree Traversals, recursion is faster than iteration in python?

I have implement tree preorder traversal in python, but found that my recursive version is faster than iteration version.

code is as below:

```
from __future__ import print_function
import time
class Tree():
def __init__(self, value):
self.value = value
self.left = None
self.right = None
def build_tree(string):
nodes = [0] + [Tree(s) for s in string]
for i in range(2, len(nodes)):
p = i/2
if i%2 == 0:
nodes[p].left = nodes[i]
else:
nodes[p].right = nodes[i]
return nodes[1]
def preorder(tree):
if tree:
# print(tree.value,end='')
preorder(tree.left)
preorder(tree.right)
def preorder2(tree):
t = tree
s = []
while t or s:
while t:
# print(t.value,end='')
s.append(t)
t = t.left
if s:
t = s.pop()
t = t.right
def main():
tree = build_tree('abcdefghi'*1000)
t = time.time()
for _ in range(100):
preorder(tree)
print(time.time() - t)
t = time.time()
for _ in range(100):
preorder2(tree)
print(time.time() - t)
if __name__ == '__main__':
main()
```

results:

```
0.751042842865
1.0220580101
```

It means recursive version is about 25% faster. I search a lot, everybody says recursive should be slower, I just wonder why it is not the case in my code?

Asked by:

**Alfred879**| Posted: 06-12-2021

# Answer 1

I believe you can simplify the iterator function and reduce the timing by eliminating one of the variables. Also, `deque`

performs better than a `set`

or a `list`

in those kinds of cases.

```
from collections import deque
def preorder3(initial_node):
queue = deque([initial_node])
while queue:
node = queue.pop()
if node.left:
queue.append(node.left)
if node.right:
queue.append(node.right)
```

The benchmarks:

```
from __future__ import print_function
from timeit import timeit
class Tree():
def __init__(self, value):
self.value = value
self.left = None
self.right = None
def build_tree(string):
nodes = [0] + [Tree(s) for s in string]
for i in range(2, len(nodes)):
p = i/2
if i%2 == 0:
nodes[p].left = nodes[i]
else:
nodes[p].right = nodes[i]
return nodes[1]
def preorder(tree):
if tree:
preorder(tree.left)
preorder(tree.right)
def preorder2(tree):
t = tree
s = []
while t or s:
while t:
s.append(t)
t = t.left
if s:
t = s.pop()
t = t.right
from collections import deque
def preorder3(initial_node):
queue = deque([initial_node])
while queue:
node = queue.pop()
if node.left:
queue.append(node.left)
if node.right:
queue.append(node.right)
tree = build_tree('abcdefghi'*100)
# Repetitions to time
number = 20
# Time it
print('preorder: ', timeit('f(t)', 'from __main__ import preorder as f, tree as t', number=number))
print('preorder2: ', timeit('f(t)', 'from __main__ import preorder2 as f, tree as t', number=number))
print('preorder3: ', timeit('f(t)', 'from __main__ import preorder3 as f, tree as t', number=number))
```

Prints:

```
preorder: 0.0256490707397
preorder2: 0.0419111251831
preorder3: 0.0269520282745
```

Answered by: **Brad275**| Posted: 07-01-2022

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