打卡帖 告别拖延症～

        dummy = ListNode(-1)
        dummy.next = head
        pre = dummy

Always bear in mind:

"use space trade for time"

        #if token != '+' and token != '-' and token != '*' and token != '/':
        if token not in ['+', '-', '*', '/']:

line 2 is much cleaner than line 1

      return '/' + '/'.join(stack)

you can always do something like this

join operation join at the end -> one/two/three...

group[key] = group.get(key, []) + [string]

B = [1,2,3] D = ['d']
B + D
-> [1,2,3,'d']

这个 数组滚动的方式值得学习
        A = [1,2]
        for i in range(n - 2):
            A.append(A[-1] + A[-2])
        
        return A[-1]

这个处理方式
https://www.jiuzhang.com/solution/climbing-stairs/

对这种题有奇怪的阴影
https://www.lintcode.com/problem/maximum-subarray/

https://www.lintcode.com/problem/subsets/

list() function used to create a new copy, as opposed to passing in the address

preoder, inorder, postorer, -> the pre is parent node,

e.g. preorder: parent node is preceding other nodes