[leetcode brush questions]

Letter combination of telephone number （ secondary ）

   def letterCombinations(self, digits):
        if len(digits)==0:
           return []

        dict={
    0:"0",1:"1",2:"abc",3:"def",4:"ghi",5:"jkl",6:"mno",7:"pqrs",8:"tuv",9:"wxyz"}
        result=[""]
        for i in digits:
            list=[]
            for j in dict[int(i)]:
                for k in result:
                    list.append(k+j)
            result=list
        return result

String multiplication （ secondary ）

def multiply(self, num1, num2):
        if num1=='0' or num2=='0':
            return '0'
        if len(num1)<len(num2):
            num1,num2=num2,num1
        num1=num1[::-1]    # Perform array transposition 
        num2=num2[::-1]
        list=[]
        for i in range(len(num2)):
            for j in range(len(num1)):
                new_num=int(num2[i]+'0'*i)
                new_num_1=int(num1[j]+'0'*j)
                list.append(new_num*new_num_1)
        ans=sum(list)
        return str(ans)