Zigzag transformation - LeetCode

 

Ideas:

 

 

code:

class Solution {
    public String convert(String s, int numRows) {
        if(numRows < 2) return s;
        List<StringBuilder> rows = new ArrayList<StringBuilder>();
        for(int i = 0; i < numRows; i++) rows.add(new StringBuilder());
        int i = 0, flag = -1;
        for(char c : s.toCharArray()) {
            rows.get(i).append(c);
            if(i == 0 || i == numRows -1) flag = - flag;
            i += flag;
        }
        StringBuilder res = new StringBuilder();
        for(StringBuilder row : rows) res.append(row);
        return res.toString();
    }
}

Code by hand:

fun convert(s: String, numRows: Int): String {
    //只能排一行一列直接返回
    if (numRows == 1 || s.length < numRows){
        return s
    }
    val res = mutableListOf<MutableList<Char>>()
    for (row in (0 until numRows)) {
        res.add(mutableListOf())
    }
    var i = 0
    var flag = 1
    for (item in s.indices) {
        res[i].add(s[item])
        if (i == numRows - 1) {
            flag = -1
        } else if (i == 0) {
            flag = 1
        }
        i += flag
    }
    return res.flatten().joinToString("")
}