二叉树的前序遍历 递归与非递归方法

常见的递归方法： 

# 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 root == None:
            return ans
        self.preTraverse(root, ans)
        return ans
    def preTraverse(self,t, ans):
        ans.append(t.val)
        if t.left:
            self.preTraverse(t.left, ans)
        if t.right:
            self.preTraverse(t.right, ans)

 非递归方法：使用的DFS做法

# 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 = []
        if root == None:
            return ans
        p = root
        while(p or len(stack)):
            if p:
                ans.append(p.val)
                stack.append(p)
                p = p.left
            else:
                p = stack[-1]
                stack.pop(-1)
                p = p.right
        return ans

参考：

https://blog.csdn.net/B08370108/article/details/88089427