gistfile1.txt

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26
Given a non-empty string containing only digits, determine the total number of ways to decode it.

Example 1:

Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).
Example 2:

Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).


keywords:

A - 1, ... Z- 26

ask: given digits, find total number of ways to decode it. 

226 -> 
first way, 2 , 2, 6 ->BBF 
22, 6 VF
2, 26, BZ

total 3 -> 

2  2     6
2 22   226
-----------  1 - 26 
B               dp[i - 1] , dp[i - 2], 22 < 26 dp[i] = dp[i - 1](i >= 1, current char is 0) + (dp[i - 2] conditional check: last two digts <= 26)

public static int FindTotalWays(string numbers)  // 226 
{
	if(numbers == null || numbers.Length == 0)  // false 
  {
  	return 0; 
  }
  
  var dp = new int[length]; // 3 
  
  for(int index = 0; index < length; index++)  // index = 1
  {   
    
  	var current = numbers[index]; // 2
    var isZero = current == '0'; // '2'
    
    // base case 
    if(index == 0)
    {
      dp[0] = isZero? 0 : 1;    // 1 
      
      if(isZero)
      {
        return 0; 
      }
    }
      
    if(index > 0 && !isZero) // true
    {
    	dp[index] += dp[index - 1]; // + dp[0] = 1
    }
    
   109
index = 2


    if(index >= 1 && numbers[index - 1] != 0 && toNumber(current, numbers[index - 1]) <= 26 && toNumber(current, numbers[index - 1]) > 0 )
    { 
      if(index == 1)
      {
      	dp[index] += 1;  // 22 ->  2 + 2
      }
      
      if(index >= 2)  
      {
     	  dp[index] += dp[index - 2]; 
      }
    }
  }
  
  return dp[length - 1]; 
}

private static int toNumber(char current, char left)
{
  var number = left - '0'; 
  number *= 10;
  number += current - '0';
  
  return number; 
  
}