LeetCode -- Decode Ways

public class Solution {
 public int NumDecodings(string s) 
 {
     if(string.IsNullOrWhiteSpace(s)){
	 	return 0;
	 }
	 
	 if(s.Length == 1){
	 	return s == "0" ? 0 : 1;
	 }
	 if(s[0] == ‘0‘ || s[0] >= ‘3‘&& s[1] == ‘0‘){
	 	return 0;
	 }
	 
	 if(s.Length == 2){
	 	return s[1] == ‘0‘ || !ok(s[0],s[1]) ? 1 : 2;
	 }
	 
	 var arr = new int[s.Length];
	 arr[0] = 1;
	  arr[1] = s[1] == ‘0‘ || !ok(s[0], s[1]) ? 1 : 2;
	 
	 for(var i = 2;i < s.Length; i++){
	 	if(s[i-1] == ‘0‘ && s[i] == ‘0‘){
			return 0;
		}
		
	 	if(s[i] != ‘0‘){
			arr[i] += arr[i-1];
		}
		if(ok(s[i-1], s[i])){
			arr[i] += arr[i-2];
		}
	 }
	 
	 return arr[s.Length-1];
 }
 
 private bool ok(char c1 , char c2){
 if(c1 == ‘0‘){
 return false;
 }
 if (c1 == ‘1‘ || c1 == ‘2‘ && c2 <= ‘6‘){
 return true;
 }
 return false;
 }
}