Leetcode 94: Binary Tree Inorder Traversal

   1
    \
     2
    /
   3

Code (recursive)

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

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root is None:
            return []
        return self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)

Code (Iterative)

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root is None:
            return []
        
        stk = []
        res = []        

        while stk or root:
            while root:
                stk.append(root)
                root = root.left
            
            root = stk.pop()
            res.append(root.val)
            root = root.right

        return res

Idea

At first, I was not sure whether to use a queue or a stack to store the nodes to be traversed. But do remember that it is an in-order traversal therefore you need to always start from the leftmost leaf. Thinking in this way, you will be easier to come to use stack.

Reference: https://discuss.leetcode.com/topic/6478/iterative-solution-in-java-simple-and-readable