Leetcode 144.先序遍历

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44. 二叉树的前序遍历

题目

给定一个二叉树，返回它的 前序 遍历。

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示例:

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

输出: [1,2,3]

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

方法

方法1：递归

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# 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 preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ans=[]
        if not root:
            return []

        ans.append(root.val)
        result=self.preorderTraversal(root.left)
        result+=self.preorderTraversal(root.right)
        return ans+result
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# 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 preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ans=[]
        def recursive(node):
            if not node:
                return None

            ans.append(node.val)
            recursive(node.left)
            recursive(node.right)
        recursive(root)
        return ans

方法2：迭代

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# 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 preorderTraversal(self, root):
        # if not root:
        #     return []
        stack = [root]
        res = []
        while stack:
            node = stack.pop()
            if node:
                stack.append(node.right)
                stack.append(node.left)
                res.append(node.val)
        return res
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# 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 preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        stack=[]
        ans=[]
        while root or stack:
            while root:
                ans.append(root.val)
                stack.append(root.right)
                root=root.left
            root=stack.pop()
        return ans
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# 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 preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ans=[]
        stack=[root]
        while stack:
            node=stack.pop()
            if not node:
                continue

            ans.append(node.val)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return ans
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# 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 preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ans=[]
        stack=[root]
        while stack:
            node=stack.pop()
            if node:
                ans.append(node.val)
                stack.append(node.right)
                stack.append(node.left)
        return ans

参考资料