LeetCode6. Z字形变换

描述

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);

Example 1:

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

Example 2:

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

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

思路

如果稍微考虑的数学一点，那么s中的第i个字符（下标从第0个开始），如果按照zigzag书写方式会出现在的行数为（行数为0到numRows-1行）：

• i % (2 * numRows - 2), if i % (2 * numRows - 2) < numRows
• 2 * numRows - 2 - (i % (2 * numRows - 2)), if i % (2 * numRows - 2) >= numRows

有了这个结果，对于任意一个位置的字符我们都知道它应该在第几行。

class Solution:

    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        # 考虑到特殊情况，即行数小于等于1或者行数大于等于字符串长度
        if numRows <= 1 or numRows >= len(s):
            return s

        arr = [''] * numRows

        for i in range(len(s)):
            tmp = i % (numRows + numRows - 2)

            if tmp < numRows:
                arr[tmp] += s[i]
            else:
                arr[numRows + numRows - 2 - tmp] += s[i]

        return ''.join(arr)

GitHub地址：https://github.com/protea-ban/LeetCode