46-Translate numbers into strings-python

Topic: Given a number, we translate it into a string according to the following rules: 0 is translated into "a", 1 is translated into "b", ..., 25 is translated into "z". There may be multiple translations for a number. Please implement a function to calculate how many different translation methods a number has.

def digit_to_string(num):
    num = str(num)
    length = len(num)
    res = [0]*length
    i=length-1
    while i>=0:
        if i==length-1:
            res[i]=1
        elif i==length-2:
            if int(num[i:])<26:
                res[i]=2
            else:
                res[i]=1
        else:
            if int(num[i:i+2]) < 26:
                res[i]=res[i+1] + res[i+2]
            else:
                res[i]=res[i+1]
        i-=1
    return res[0]

　　Note: Use the idea of ​​dynamic programming. f[i]=f[i+1]+g[i,i+1]f[i+2], g[i,i+1] is whether the number composed of i, i+1 is less than 26, If it is less than, it can be split into two, g[i,i+1] is 1, if it is greater than or equal to 26, it cannot be split, and g[i,i+1] is 0.