Difficulty
Medium.
Problem
Given a binary tree, return the preorder traversal of its nodes' values.
Example:
Input: [1,null,2,3]
1
\
2
/
3
Output: [1,2,3]
Follow up: Recursive solution is trivial, could you do it iteratively?
AC
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
# DFS
class Solution():
def preorderTraversal(self, root):
res = []
self.preorder(root, res)
return res
def preorder(self, root, res):
if root:
res.append(root.val)
self.preorder(root.left, res)
self.preorder(root.right, res)
# Stack
class Solution():
def preorderTraversal(self, root):
res, stack = [], [(root, False)]
while stack:
root, visited = stack.pop()
if not root:
continue
if visited:
res.append(root.val)
else:
stack.append((root.right, False))
stack.append((root.left, False))
stack.append((root, True))
return res