3.1 leetcode daily question 6

The first feeling is to use a two-dimensional array to store each character , It's good to find a rule, but it doesn't seem to find it …

character	Indexes
P	0
A	1
Y	2
P	3
A	4
L	5
I	6
S	7
H	8
I	9
R	10
I	11
N	12
G	13

If numRows=4
The first 0 That's ok -> P - I - N
Corresponding index -> 0 - 6 -12
spacing -> 6 - 6

The first 1 That's ok -> A - L - S - I - G
Corresponding index -> 1 - 5 - 7 - 11 - 13
spacing -> 4 - 2 - 4 - 2

The first 2 That's ok -> Y - A - H - R
Corresponding index -> 2 - 4 - 8 - 10
spacing -> 2 - 4 - 2

The first 3 That's ok -> P - I
Corresponding index -> 3 - 9
spacing -> 6

It's not hard to see. , The rule is in the spacing , set up step = 2*numRows - 2, When numRows = 4 when ,step = 6
And the index corresponding to the first element of each row is the row number
The spacing between the elements in the first and last lines is step
The spacing of elements in other lines is step-2* Row number And 2* Row number In exchange for

public static String convert(String s, int numRows) {
    
        if (numRows == 1) {
    
            return s;
        }

        List<StringBuilder> rows = new ArrayList<>();
        // stay List Add... To the collection numRows individual StringBuilder object 
        for (int i = 0; i < numRows; i++) {
    
            rows.add(new StringBuilder());
        }

        int step = 2 * numRows - 2;

        for (int i = 0; i < numRows; i++) {
    
            int index = i;
            int add = i * 2;
            if (i == 0) {
    
                while (index < s.length()) {
    
                    rows.get(i).append(s.charAt(index));
                    index += step;
                }
            } else if (i == numRows - 1) {
    
                while (index < s.length()) {
    
                    rows.get(i).append(s.charAt(index));
                    index += step;
                }
            } else {
    
                while (index < s.length()) {
    
                    rows.get(i).append(s.charAt(index));
                    add = step - add;
                    index = index + add;
                }
            }
        }
        StringBuilder res = new StringBuilder();
        for (StringBuilder row : rows) {
    
            res.append(row);
        }
        return res.toString();
    }

Then I saw the thought of a big man , Because the characters turn on the zero line and the last line , Let's play one. flag Enable it to turn

public static String convert(String s, int numRows) {
    
        if (numRows == 1) {
    
            return s;
        }
        List<StringBuilder> rows = new ArrayList<>();
        for (int i = 0; i < numRows; i++) {
    
            rows.add(new StringBuilder());
        }

        int i = 0;
        int flag = -1;
        for (char c : s.toCharArray()) {
    
            rows.get(i).append(c);
            if (i == 0 || i == numRows - 1) {
    
                flag = -flag;
            }
            i = i + flag;
        }

        StringBuilder res = new StringBuilder();
        for (StringBuilder row : rows) {
    
            res.append(row);
        }
        return res.toString();
    }