LeetCode - ZigZag Conversion

Problem description

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 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

Analysis

Basic idea is to construct the grid, first divide the grid into several groups. Like this:

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

And the group contains numRows * 2 - 2 character in string, and the width of the grid should be numGroups * (numRows - 1), and we can construct the grid and use a StringBuilder to get the output.

Here’s the code:

public class ZigZagConversion {
  public String convert(String s, int numRows) {
    int len = s.length();

    // miss this part
    if (numRows == 1){
      return s;
    }

    int group = numRows * 2 - 2;

    int numGroups = len / group + 1;
    int width = numGroups * (numRows - 1);

    char[][] grid = new char[numRows][width];

    for (int i = 0; i < numRows; i++) {
      Arrays.fill(grid[i], ' ');
    }

    int index = 0;
    for (int i = 0; i < numGroups; i++) {
      int innerIndex = 0;
      while (index < s.length() && innerIndex < group) {
        char c = s.charAt(index);
        int col = i * (numRows - 1);
        if (innerIndex < numRows) {
          grid[innerIndex][col] = c;
        } else {
          int j = innerIndex - numRows + 1;
          grid[numRows - 1 - j][col + j] = c;
        }
        index++;
        innerIndex++;
      }
    }

    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < numRows; i++) {
      for (int j = 0; j < width; j++) {
        if (grid[i][j] != ' ') {
          sb.append(grid[i][j]);
        }
      }
    }

    return sb.toString();
  }
}

A better solution without extra space but same logic is:

class Solution {
    public static String convert(String s, int numRows) {
        if(numRows == 1)
            return s;
        StringBuilder res = new StringBuilder();
        for (int i = 0; i < numRows; i++) {
            int first = (numRows - i) * 2 - 2;
            int second = i * 2;
            int flag = 0;
            if (i == 0) {
                for (int j = i; j < s.length(); j += first)
                    res.append(s.charAt(j));
            } else if (i == numRows - 1) {
                for (int j = i; j < s.length(); j += second)
                    res.append(s.charAt(j));
            } else {
                for (int j = i; j < s.length(); ) {
                    res.append(s.charAt(j));
                    if (flag == 0) {
                        j += first;
                        flag = 1;
                    } else {
                        j += second;
                        flag = 0;
                    }
                }
            }
        }
        return res.toString();
    }
}

Another simple solution is to use several StringBuilder to indicate different rows and add character from top to bottom and bottom to top.

Here’s the main code:

String test(String s, int numRows){
    if (numRows == 1){
      return s;
    }
    StringBuilder[] sbs = new StringBuilder[numRows];
    for(int i = 0; i < numRows; i++){
      sbs[i] = new StringBuilder();
    }

    boolean flag = true; // true for top to bottom, else for bottom to top
    int index = 0;
    for(char c:s.toCharArray()){
      if (flag){
        sbs[index++].append(c);
        if (index == numRows){
          index-=2;
          flag = false;
        }
      }
      else {
        sbs[index--].append(c);
        if (index < 0){
          index+=2;
          flag = true;
        }
      }
    }

    String res = "";
    for(StringBuilder sb:sbs){
      res+=sb.toString();
    }

    return res;
  }

What to improve

• be careful about divide by zero exception