[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

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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)

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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

 

类似题目：

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