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6. Zigzag Conversion

Medium

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

Example 3:

Input: s = “A”, numRows = 1

Output: “A”

Constraints:

• 1 <= s.length <= 1000
• s consists of English letters (lower-case and upper-case), ',' and '.'.
• 1 <= numRows <= 1000

Solution

using System.Text;

public class Solution {
    public string Convert(string s, int numRows) {
        int sLen = s.Length;
        if (numRows == 1) {
            return s;
        }
        int maxDist = numRows * 2 - 2;
        StringBuilder buf = new StringBuilder();
        for (int i = 0; i < numRows; i++) {
            int index = i;
            if (i == 0 || i == numRows - 1) {
                while (index < sLen) {
                    buf.Append(s[index]);
                    index += maxDist;
                }
            } else {
                while (index < sLen) {
                    buf.Append(s[index]);
                    index += maxDist - i * 2;
                    if (index >= sLen) {
                        break;
                    }
                    buf.Append(s[index]);
                    index += i * 2;
                }
            }
        }
        return buf.ToString();
    }
}

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