Leetcode binary tree class

114 topic ： Binary tree preorder traversal

Iterative method + Stack , Binary tree to achieve the preorder traversal

class Solution(object):
    def preorderTraversal(self, root):
        # Stack + Iterative implementation 
        # Pressing stack append, Out of the stack pop
        if not root:
            return []
        stack=[root]
        result=[]
        while stack:
            node=stack.pop()# Processing node 
            result.append(node.val)
            # Right then left , Leave the stack first and then right 
            if node.right!=None:
                stack.append(node.right)
            if node.left!=None:
                stack.append(node.left)
        return result

226 topic ： Flip binary tree

Recursive Trilogy
1、 Determine the parameters and return values of recursive functions
2、 Determine termination conditions
3、 Determine the logic of monolayer recursion

Just flip the left and right nodes of each child

class Solution(object):
    def invertTree(self, root):
        if not root:
            return None

        root.left,root.right=root.right,root.left
        self.invertTree(root.left)
        self.invertTree(root.right)
        return root