ZigZag Conversion - LeetCode JavaScript

題目

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".

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題目解釋

假設咱們輸入的就是上面的字符串,在numRows取不一樣值的時候,你們看我用Excel作的簡易的圖:

看完圖應該就明白了這個題目究竟是什麼意思,這個題的難點在於找規律,只要找到了規律,解決這個題並不難.
規律以下:

• 第一行或者最後一行,元素之間的間隔(distance)爲
  2 * nRows - 2;
• 中間部分間隔分別是:
  distace - 2i, 2 i, distance - 2i, 2i...

  依照上面規律,代碼以下

/**
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */
var convert = function(s, numRows) {
    if (numRows <= 1)
        return s;
    let res = '';
    let len = s.length;
    let dis = numRows*2 - 2;
    
    /* first row */
    for (let i = 0; i < len; i+=dis) {
        res += s.charAt(i);
    }
    
    /* middle */
    for (let i = 1; i < numRows-1; i++) {
        let tempDis = 2*i;
        for (let j = i; j < len; j += tempDis) {
            res += s.charAt(j);
            tempDis = dis - tempDis;
        }
    }
    
    /* last row */
    for (let i = numRows-1; i< len; i += dis) {
        res += s.charAt(i);
    }
    return res;
};