ZigZag conversion

Solution.cs

using System.Text;
using System.Collections.Generic;

public class Solution {
  public string Convert(string s, int numRows) {
    
    StringBuilder sb = new StringBuilder();
    List<int> pillarIndexes = new List<int>();
    int i = 0;
    int h = numRows - 1;
    
    while(i < s.Length)
    {
      pillarIndexes.Add(i);
      if(h == 0) i++;
      else
      {
        i += h; // pillar
        i += h; // diagnol connector
      }
    }
    
    for(i = 0; i <= h; i++)
    {
      for(int j = 0; j < pillarIndexes.Count; j++)
      {
        if((pillarIndexes[j] + i) >= s.Length) break;
        sb.Append(s[pillarIndexes[j] + i]);
        if(i > 0 && (h - i) > 0)
        {
          var pe = j == 0 ? h : (pillarIndexes[j] + h);
          var di = pe + h - i;
          if(di >= s.Length) break;
          sb.Append(s[di]);
        }
      }
    }
    
    
    return sb.ToString();
  }
}