Project Euler：Problem 81 Path sum: two ways

In the 5 by 5 matrix below, the minimal path sum from the top left to the bottom right, by only moving to the right and down, is indicated in bold red and is equal
to 2427.

⎛⎝⎜⎜⎜⎜⎜⎜131201630537805673968036997322343427464975241039654221213718150111956331⎞⎠⎟⎟⎟⎟⎟⎟

Find the minimal path sum, in matrix.txt (right
click and "Save Link/Target As..."), a 31K text file containing a 80 by 80 matrix, from the top left to the bottom right by only moving right and down.

很显然要动态规划

除去矩阵最上面一行和最左边一行，dp[i][j]=min(dp[i+1][j],dp[i][j+1])

ls=[]
for line in open("matrix.txt"):
    #print(line)
    a=line.split(',')
    a=[int(i) for i in a]
    ls.append(a)

#先计算第一行和第一列
for i in range(78,-1,-1):
    ls[79][i]=ls[79][i]+ls[79][i+1]
    ls[i][79]=ls[i][79]+ls[i+1][79]

for i in range(78,-1,-1):
    for j in range(78,-1,-1):
        ls[i][j]=ls[i][j]+min(ls[i+1][j],ls[i][j+1])

print(ls[0][0])