This is a problem I found by accident , It bothered me for half an hour ：
Sometimes we will find out , Assign a value to a position of a two-dimensional array , But it directly changes the value of an entire column ：
- This is because we are defining dp When used, it's like this ：dp = n*[n*[0]]
- n individual list Shallow copy connection of , Modifying any of these elements will change the entire list
We can change it to ：dp = [[0 for i in range(n)] for i in range(n)] Problem solvable 
