006-zigzag-conversion

Question

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

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)

Example:

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:

convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".

Thought Process

1. Magic Number
   1. The first row and last row always insert next 2 * num - 2th position, or referred as num in the code (excluding the character in the middle)
   2. In the middle, we increase the next position by alternating between diff and num - diff
   3. Time complexity O(n)
   4. Space complexity O(n)

Solution

class Solution {
    public String convert(String s, int numRows) {
        if(numRows==1) return s;
        char[] chars = s.toCharArray();
        int n = chars.length;
        StringBuilder sb = new StringBuilder();
        int num = numRows * 2 - 2;
        for(int i = 0; i < numRows; i++){
            int j = i;
            int diff = i * 2;
            while(j < n){
                sb.append(chars[j]);
                diff = (diff == num) ? num : num - diff; 
                j += diff;
            }
        }
        return sb.toString();
    }
}

Additional