LeetCode #144 Binary Tree Preorder Traversal

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Problem

Given a binary tree, return the preorder traversal of its nodes’ values.

Example

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Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

Boundary Conditions

  • Will input list node be null?

Solution1

Method: Depth-first Search (Recursive)

Time Complexity: O(n)

Space Complexity: O(n)

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def preorderTraversal(self, root, ret = []):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        if not ret:
            ret = []
        
        ret.append(root.val)
        self.preorderTraversal(root.left, ret)
        self.preorderTraversal(root.right, ret)
        
        return ret

or

Solution2

Method: Depth-first Search (Iterative)

Time Complexity: O(n)

Space Complexity: O(n)

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        
        ret = []
        stack = [root]
        
        while stack:
            node = stack.pop()
            if node:
                ret.append(node.val)
                stack.append(node.right)
                stack.append(node.left)
        
        return ret