[LeetCode] 144. Binary Tree Preorder Traversal 二叉树的先序遍历

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

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

树的遍历，最常见的有先序遍历，中序遍历，后序遍历和层序遍历，它们用递归实现起来都非常的简单。而题目的要求是不能使用递归求解。

1. 用迭代和stack。2. Morris Traversal Solution

Python: Stack,  Time: O(n), Space: O(h) # h is the height of the tree

class Solution2(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        result, stack = [], [(root, False)]
        while stack:
            root, is_visited = stack.pop()
            if root is None:
                continue
            if is_visited:
                result.append(root.val)
            else:
                stack.append((root.right, False))
                stack.append((root.left, False))
                stack.append((root, True))
        return result

Python: Morris， Time: O(n), Space: O(1)

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        result, curr = [], root
        while curr:
            if curr.left is None:
                result.append(curr.val)
                curr = curr.right
            else:
                node = curr.left
                while node.right and node.right != curr:
                    node = node.right
            
                if node.right is None:
                    result.append(curr.val)
                    node.right = curr
                    curr = curr.left
                else:
                    node.right = None
                    curr = curr.right
                
        return result　　

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[LeetCode] 94. Binary Tree Inorder Traversal 二叉树的中序遍历