Leetcode 6. ZigZag conversion

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 text, int nRows);

convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".

 

思路： 找出string中第i个元素所处的位置，这里假设numRows = 4。

0 6

1  5 7

2  4 8

3 9

我们可以观察到的是每一个block有2*numRows - 2 = 6个元素。 p元素所在的行数就是p if p < numRows, and numRows - 2 - (p- numRows) if p = i % block。本题的OJ有点问题，如果用二维矩阵的话，似乎无法通过。

def convert(s, numRows):
    """
    :type s: str
    :type numRows: int
    :rtype: str
    """
    if numRows == 1:
        return s
    blockSize = 2 * numRows - 2
    mat = [[] for row in range(numRows)]

    for i, elem in enumerate(s):
        p = i % blockSize
        if p < numRows:
            mat[p].append(elem)
        else:
            mat[numRows - 2 - (p - numRows)].append(elem)

    res = []
    for i in range(numRows):
        res += mat[i]
    res = ''.join(res)

    return res