Zigzag Pattern String Conversion - Leet Code Solution

Problem Statement

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:

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

Solution

If you look the pattern, its going in two directions:

Top to bottom straight
Bottom to top with one incremented index

You need one flag to indicate whether you are going downward or going upward.

Algorithm

Maintain a flag to indicate whether you are going upward or downward. Reset boundaries are on 0-index and length-1 index.
Toggle the flag on reaching boundaries (top most, or bottom most)
Maintain a List of string(StringBuffer in Java). List length is equal to the number of rows asked.
Maintain a variable which will tell you in which stringbuffer(row num) to add the current character

Code

public class Q6_ZigZagConversion {
	private String str;
	
	public Q6_ZigZagConversion(String str) {
		this.str = str;
	}
	
	public String conversion(int numRows) {
		int l = this.str.length();
		List<StringBuffer> zigzag = new ArrayList<StringBuffer>();
		for (int i=0; i<numRows; i++) {
			zigzag.add(new StringBuffer());
		}
		
		boolean comingFromTop = true;
		int zig = 0;
		for (int i=0; i<l; i++) {
			zigzag.get(zig).append(this.str.charAt(i));
			
			if (zig == numRows-1) {
				comingFromTop = false;
			}
			else if (zig == 0) {
				comingFromTop = true;
			}
			zig = comingFromTop ? zig + 1 : zig - 1;
			zig = zig % numRows;
		}
		
		StringBuffer sb = new StringBuffer();
		for (int i=0; i<numRows; i++) {
			sb.append(zigzag.get(i));
		}
		
		return sb.toString();
	}
}