Description:
The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string s, int numRows);
Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"

Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

Step 1 : 標記 每個字母出現的位置

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        slen =len(s)
        pos = []
        row = 1
        flag = 0 
        for i in range(slen):
               
            if row==numRows:
                #print("z")
                #row-=1
                #print("r:"+str(row))
                pos.append(row)
                row = row -1
                flag = 1 # go up
            elif row==1:
                pos.append(row)
                row = row +1 
                flag = 0 
            else:
                pos.append(row)
                if flag == 0:
                    row = row +1
                else:
                    row = row -1
        print(pos)        

Step 2-1 : 顯示出來

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        slen =len(s)
        pos = []
        row = 1
        flag = 0 
        for i in range(slen):
               
            if row==numRows:
                #print("z")
                #row-=1
                #print("r:"+str(row))
                pos.append(row)
                row = row -1
                flag = 1 # go up
            elif row==1:
                pos.append(row)
                row = row +1 
                flag = 0 
            else:
                pos.append(row)
                if flag == 0:
                    row = row +1
                else:
                    row = row -1
        print(pos)
        
        ans =""
        for i in range(numRows): # sum of round
            for j in range(len(s)):
                if pos[j] == (i+1):
                    ans+=s[j]
        print(ans)
        return ans

Step 2-2 : 沒有這步 會過不了 numRows 為1的情況
先判斷 numRows是否是 1 ，是的話，直接回傳 input 字串

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        slen =len(s)
        pos = []
        row = 1
        flag = 0
        
        if numRows==1:
            return s
        
        for i in range(slen):  
            if row==numRows:
                #print("z")
                #row-=1
                #print("r:"+str(row))
                pos.append(row)
                row = row -1
                flag = 1 # go up
            elif row==1:
                pos.append(row)
                row = row +1 
                flag = 0 
            else:
                pos.append(row)
                if flag == 0:
                    row = row +1
                else:
                    row = row -1
        print(pos)
        
        ans =""
        for i in range(numRows): # sum of round
            for j in range(len(s)):
                if pos[j] == (i+1):
                    ans+=s[j]
        print(ans)
        return ans        

Result