Interview Question ZigZag Conversion

By Fatih KABAKCI

The problem is to convert a string to a new pattern which is called zigzag that shapes characters in rows. Algorithm starts to run by asking the user that how many rows the user would like to input, then it converts the string a zigzag pattern that split all characters in rows.

Problem #6 - 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).

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:.

Difficulty: Easy

Solution in Java

The solution can be implemented as imagining of the variable deep travels like a sinusoidal curve as increasing first from 0 to number of rows, and then again it decreases from number of rows to 0. Then, each character is stored in the related deep level during this period that looks like below.

package com.fatihkabakci.Easy.Zigzag;

/**
 * 
 * @author fkabakci
 * Problem 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)
 * 
 *  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 text, int nRows);
 *  
 *  convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".
 * 
 * Examples:
 * 
 * Input: "PAYPALISHIRING", 3
 * Output: "PAHNAPLSIIGYIR"
 * 
 * Input: "ABCDE", 3
 * Output: "AEBDC"
 * 
 * Input: "ABCDEFG", 2
 * Output: "ACEGBDF"
 * 
 * Solution:
 *
 * 1. Create 2D array to distinguish characters each other depending on which deep they belong to.
 * 2. Create index array whose length is as number of numRows to track each deep character array.
 * 3. The variable deep always travels from 0 to numRows - 1 and from numRows - 1 to 0 during the loop.
 * 4. Get each character from string and save into 2D array which stores characters with deep-element.
 * 5. Merge all elements ordering by deep level into a string and return it.
 */
public class ZigZagConversion {
	public String convert(String s, int numRows) {
		if(numRows < 2)
			return s;
		
		int deep = 0, n = s.length();
		// elements table, deep number - indexes[0],[1],[2] elements
		char[][] t = new char[numRows][n];
		// number of numRows indexes which are counter for deep elements
		int[] indexes = new int[numRows];
		boolean UP = false, DOWN = true, direction = DOWN; // start with down
		
		for (int i = 0; i < n; i++) {
			if (deep == 0)
				direction = DOWN;
			else if (deep == numRows - 1)
				direction = UP;

			t[deep][indexes[deep]++] = s.charAt(i);

			if (direction)
				deep++;
			else
				deep--;
		}

		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < numRows; i++) {
			for (int j = 0; j < indexes[i]; j++) {
				sb.append(t[i][j]);
			}
		}
		return sb.toString();
	}

	public static void main(String[] args) {
		ZigZagConversion z = new ZigZagConversion();
		System.out.println(z.convert("PAYPALISHIRING", 3));
	}
}

The algorithm time complexity is O(n). Because of 2D array, space complexity is O(n^2).