leetcode226 Invert Binary Tree

"""
Invert a binary tree.
Example:
Input:
     4
   /   
  2     7
 /    / 
1   3 6   9
Output:
     4
   /   
  7     2
 /    / 
9   6 3   1
"""
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution1(object):
    def invertTree(self, root):
        if root == None:
            return root
        root.left, root.right = root.right, root.left
        self.invertTree(root.left)
        self.invertTree(root.right)
        return root
"""
递归都可以用栈来迭代实现
因此存在用栈遍历的第二种解法
"""
class Solution2(object):
    def invertTree(self, root):
        if not root:
            return root
        stack = [root]
        while stack:
            node = stack.pop(-1)  #先弹出根结点，再依次让左右孩子压栈
            if node.left:
                stack.append(node.left) #pop(0)是弹出list的头元素 pop(-1)弹出尾元素
            if node.right:              #在这里用两个均可，即队列和栈都可以
                stack.append(node.right)
            node.left, node.right = node.right, node.left
        return root