6.Z 字型变换

import java.util.ArrayList;
import java.util.List;
 
public class Q6 {
    public String convert(String s, int numRows) {
        if (numRows == 1) return  s;
 
        List<StringBuilder> rows = new ArrayList<>();
        // numRows行字符串，使用StringBuilder方便后续的修改
        for (int i = 0; i < Math.min(numRows,s.length()); i++) {
            rows.add(new StringBuilder());
        }
 
        int curRow  = 0;
        boolean  goingDown = false;
 
        //字符的规律，从顶部到底部，然后改变方向，从底部到顶部，再改变方向。
        for (char c :s.toCharArray()) {
            rows.get(curRow).append(c);
            //第一行或最后一行 改变方向
            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();
 
 
    }
}