Z字形输出字符串

class Solution:
    def zconvert(self , str: str, k: int) -> str:
        res=""
        for i in range(k):
            for j in self.getNum(i,k,len(str)):
                res+=str[j]
        return res
            
    def getNum(self,i,k,l):
        delta=2*(k-1)
        p=i
        while True:
            yield p
            p=int(p/delta+1.5)*delta-p%delta
            if p>=l:
                break