Day 12 - LeetCode 6. ZigZag Conversion

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".

翻譯

字串"PAYPALISHIRING"經過Z字轉換後如圖所示，重組後變成"PAHNAPLSIIGYIR"， 寫一個convert(string text, int nRows)來將傳入的字串text轉換成n行的Z字轉換。
convert("PAYPALISHIRING", 3) 會回傳 "PAHNAPLSIIGYIR"。

這邊用另外一個範例來解釋會比較清楚：
text = "ABCDEFGHIJKLMN", n = 4，排成Z字如下，因此轉換後的字串為 "AGMBFHLNCEIKDJ"

A     G     M
B   F H   L N
C E   I K   
D     J  

Z字行轉換也就是波浪

想法

建立對應 numRows 儲存位置空陣列, 設立一個指標控制儲存順序, 模擬上下波浪效果

詳細參閱

https://discuss.leetcode.com/topic/80083/intuitive-javascript-solution

.reduce用法
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce
https://aotu.io/notes/2016/04/14/js-reduce/
https://msdn.microsoft.com/zh-cn/library/ff679975(v=vs.94).aspx

.concat用法
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

---

Code

/**
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */
var convert = function(s, numRows) {

    if (s.length < numRows || numRows ===1) {
        return s;
    }

    let rows = [];
    let reverse = false;
    let count = 0;
    let ans = '';

    //建立儲存位置空陣列
    for (let i = 0; i < numRows; i++) {
        rows[i] = [];
    }

    for (let i = 0; i < s.length; i++) {
        //依位置儲存字串
        rows[count].push(s[i]);

        if ( !reverse) {
            count++;
        } else {
            count--;
        }

        if (count === numRows-1 || count === 0) {
            //狀態相反, 控制寫入陣列位置
            reverse = !reverse;
        }
    }
    //reduce 方法將數組中的每一個值與前面的被返回相加的總和(初試值為數組的第一個值或者initialValue)
    ans = rows.reduce(function(pre, cur){
        //.concat 合併陣列, 產生一個新陣列
        return pre.concat(cur);
    });

    return ans.join('');
};

Run

Your input
"", 1
"QWERTYUIAS", 3
"JKHJGFDFSSUIYOIYTRDECVB", 4

Your answer
""
"QTAWRYISEU"
"JDYDKFFIOREHGSUITCBJSYV"

Expected answer
""
"QTAWRYISEU"
"JDYDKFFIOREHGSUITCBJSYV"

Runtime: 92 ms