leetcode6：Z字形变换

=============================Python==========================

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        res = ['' for _ in range(numRows)]
        i = 0
        while i < len(s):
            j = 0
            while i < len(s) and j < numRows:
                res[j] += s[i]
                i += 1
                j += 1
            k = numRows - 2
            while i < len(s) and k > 0:
                res[k] += s[i]
                i += 1
                k -= 1
        ans = ''
        for i in res:
            ans += i
        return ans

==============================Java===========================

class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1) return s;
        List<StringBuilder> rows = new ArrayList<>();
        for (int i = 0; i < Math.min(numRows, s.length()); i++){
            rows.add(new StringBuilder());
        }
        int curRow = 0;
        boolean goingDown = false;
        for (char c : s.toCharArray()) {
            rows.get(curRow).append(c);
            if (curRow == 0 || curRow == numRows - 1) {
                goingDown = !goingDown;
            }
            curRow += goingDown ? 1 : -1;
        }

        StringBuilder ret = new StringBuilder();
        for (StringBuilder row : rows) ret.append(row);
        return ret.toString();

    }
}