尝试将字符附加到字符串时，“+：NoneType 不支持的操作数类型”

Question

I am trying to solve the following question:

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)

P   A   H   N
A P L S I I G
Y   I   R

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);

I have written the following code but I am getting error in the bold line " TypeError: unsupported operand type(s) for +: 'NoneType' and 'unicode'"

class Solution(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        if numRows==1:
            return s
        
        templist=[None]*numRows
        ret=" "
        
        curRow=0
        goingDown=0
        for each_char in s:
            if templist[curRow]:
                templist[curRow]=each_char
            else:
                **templist[curRow]=templist[curRow] + each_char**
            if (curRow==numRow-1 or curRow == 0):
                goingDown = not goingDown
            if goingDown:
                curRow=curRow+1
            else:
                curRow=curRow-1
                
        for each_str in templist:
            ret=ret+each_str
        print ret

Am I doing something wrong in this. It will be great if someone can point out the problem here. Thanks in advance

Answer 1

Your conditions seem to be reversed in the following lines:

        if templist[curRow]:
            templist[curRow]=each_char
        else:
            **templist[curRow]=templist[curRow] + each_char**

It should probably read:

        if templist[curRow]:
            templist[curRow]=templist[curRow] + each_char
        else:
            templist[curRow]=each_char

This ensures that it only appends to the string in templist[curRow] if that string already exists (is not None ), and so avoids the error of adding a string to None .

A better way might be to set templist = [""] * numRows , ie a list of empty strings, then just add to it using templist[curRow] += each_char , which will always work because you can add a character to an empty string.

Answer 2

Yes, you're doing something wrong.

The line templist=[None]*numRows makes your templist variable hold a bunch of None s. You then proceed to take one of those None s, and try to "add" one of them to a string with the statement that you have in bold: templist[curRow]=templist[curRow] + each_char (ie the righthand side of this assignment evaluates to None + each_char ).

You can't add a string and a None in Python, hence the error.

Answer 3

string = 'PAHNAPLSIIGYIR'
string = list(string)
rows = []

def make_output(string, num_rows):
    i = 0
    while i < num_rows:
        if i%2 == 0:
            a = []
            while len(a) < 4:
                if len(string)==0:
                    break
                a.append(string.pop(0)) 
            rows.append(a)
        if i%2 != 0:
            a = []
            while len(a) < 7:
                a.append(string.pop(0)) 
            rows.append(a)
        i += 1

    for i in range(len(rows)):
        if i%2 == 0:
            print('   '.join(rows[i]))
        if i%2 != 0:
            print(' '.join(rows[i]))

make_output(string,  3)  

You can use a function like this, where you designate lengths for even and odd rows, then you can pop from your string. Then you can just print even and odd rows with proper spacing.

 (xenial)vash@localhost:~/python/AtBS$ python3.7 solve.py PAHN APLSIIG YIR (xenial)vash@localhost:~/python/AtBS$ python3.7 solve.py MM * H YAESVS * NIA