LeetCode 94. Binary Tree Inorder Traversal

Description

https://leetcode.com/problems/binary-tree-inorder-traversal/

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?

Explanation

One method to solve this problem is using recursion. We can define a helper function to implement recursion.

Python Solution

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

class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        result = []
        
        self.helper(root, result)
        
        return result
    
    
    def helper(self, root, result):
        if root == None:
            return
        
        self.helper(root.left, result)
        result.append(root.val)
        self.helper(root.right, result)
  • Time complexity: O(N).
  • Space complexity: O(N).

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