2020/1/20 leetcode04

def convert(self,s:str,numRows:int) ->str:
    if numRows<2:return s
    res=["" for _ in range(numRows)]
    i,flag=0,-1
    for c in s:
        res[i]+=c
        if i ==0 or i ==numRows -1:flag=-flag
        i+=flag
     return "".join(res)