144. 二叉树的前序遍历

方法一：迭代代码一：

class Solution(object):
    # 迭代
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        pre_stack = [root]
        ans = []
        while pre_stack:
            cur = pre_stack.pop()
            ans.append(cur.val)
            if cur.right:
                pre_stack.append(cur.right)
            if cur.left:
                pre_stack.append(cur.left)
        return ans

迭代代码二：

class Solution(object):
    # 迭代
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        pre_stack = []
        ans = []
        cur = root
        while pre_stack or cur:
            while cur:
                ans.append(cur.val)
                pre_stack.append(cur)
                cur = cur.left
            temp = pre_stack.pop()
            cur = temp.right
        return ans

方法二：递归

class Solution(object):
    # 递归
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right)