94. Binary Tree Inorder Traversal

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Problem

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Given a binary tree, return the inorder traversal of its nodes' values.

Example:

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

Output: [1,3,2]
Follow up: Recursive solution is trivial, could you do it iteratively?

Solution

Sol 1 Recursion

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        L = []
        def helper(root):
            if root == None:
                return
            helper(root.left)
            L.append(root.val)
            helper(root.right)
        helper(root)
        return L

Sol 2 Iteration

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        L = []
        stack = []
        while stack or root:
            while root:
                stack.append(root)
                root = root.left
            root = stack.pop()
            L.append(root.val)
            root = root.right
        return L