post order
post order is reverse of pre order, when appending value, simply add it to the front of the result list instead of the end. 

class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res=[]
        stack=[root]
        while stack:
            node=stack.pop()
            if node:
                res.insert(0,node.val)
                stack.append(node.left)
                stack.append(node.right)
        return res

in order
class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res=[]
        stack=[]
        cur=root
        while stack or cur:
            #add all possible left node into the stack 
            while cur:
                stack.append(cur)
                cur=cur.left
            #start adding to result from the left most node 
            cur=stack.pop()
            res.append(cur.val)
            #after reading the root node, start reading the right subtree 
            cur=cur.right
        return res

pre order
class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        #use the LIFO feature of stack,
        #if the node is not empty, output the value,  then first put right node into stack, then put left node into stack
        #next loop will read the left node first, after finish interating the left node, it will pop the right node and iterate the right node. 
        res=[]
        stack=[root]
        while stack:
            node=stack.pop()
            if node:
                res.append(node.val)
                stack.append(node.right)
                stack.append(node.left)
                
        return res