https://www.youtube.com/watch?v=8zXTAn7i5rU


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 s, int numRows);
Example 1:
Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"
Example 2:
Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

P     I    N
A   L S  I G
Y A   H R
P     I




Create nRows StringBuffers, and keep collecting characters from original string to corresponding StringBuffer. Just take care of your index to keep them in bound.

public String convert(String s, int nRows) {
    char[] c = s.toCharArray();
    int len = c.length;
    StringBuffer[] sb = new StringBuffer[nRows];
    for (int i = 0; i < sb.length; i++) sb[i] = new StringBuffer();
    
    int i = 0;
    while (i < len) {
        for (int idx = 0; idx < nRows && i < len; idx++) // vertically down
            sb[idx].append(c[i++]);
        for (int idx = nRows-2; idx >= 1 && i < len; idx--) // obliquely up
            sb[idx].append(c[i++]);
    }
    for (int idx = 1; idx < sb.length; idx++)
        sb[0].append(sb[idx]);
    return sb[0].toString();
}




// wrong answer 

class Solution {
    public String convert(String s, int numRows) {
        char[] c = s.toCharArray();
        int len = c.length;
        StringBuilder[] sb = new StringBuilder[numRows];
        for(int i = 0; i < sb.length; i++) sb[i] = new StringBuilder();
        
        int i = 0;
        while(i < len){
            // vertically down 
            // index is the which row 
            for(int index = 0; i < numRows && i < len; index++){
                sb[index].append(c[i++]);
            }
            // obliquely up 
            for(int index = numRows - 2; index >= 1 && i < len; index--){
                sb[index].append(c[i++]);
            }
        }
        for(int index = 1; index < sb.length; index++){
            sb[0].append(sb[index]);
        }
        return sb[0].toString();
        
    }
}