# 6. ZigZag Conversion (Medium)

https://leetcode.com/problems/zigzag-conversion/

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```

## Solutions

``````class Solution {

// Zigzag is a pattern that repeats every numRows * 2 - 2 chars. To find out the
// pattern, put the elements into a matrix.

// The the row size is numRows, the columns size
// should be (s.length() / (numRows * 2 - 2) + numRows - 1) * (numRows-1), where
// s.length() / (numRows * 2 - 2) + 1 is the number of pattern groups.

// Literally, adding 1 or not depends on specific situation. But to make it easier, I always advise
// to add; numRows - 1 is the number of chars in each ziazag pattern.

public String convert(String s, int numRows) {
String ans = "";

if (s == null || s.length() == 0) {
return ans;
}

if (numRows == 1) {
return s;
}

int len = s.length();

int groupSize = 2 * numRows - 2;
for (int i = 0; i < numRows; i++) {
for (int j = i; j < len; j += groupSize) {
ans += s.charAt(j);

// Make sure char index is within the boundary.
if (i % (numRows - 1) != 0 && j + groupSize - 2 * i < len) {
ans += s.charAt(j + groupSize - 2 * i);
}
}
}

return ans;
}
}
``````