Sum of Digits of String After Convert

Problem Statement

In this task, you receive a string s consisting solely of lowercase English letters, along with an integer k. The primary goal is to convert this string into a numeric value uniquely and then modify this numerical representation by summing its digits repeatedly for k iterations. The conversion and transformation process involves:

1. Convert characters to numbers: Replace each character with its corresponding position in the alphabet (i.e., 'a' becomes 1, 'b' becomes 2, ..., 'z' becomes 26). Concatenate these values into a single string of digits.
2. Transform the result k times: Take the string of digits and repeatedly sum all the digits k times. After each sum, replace the string with the new sum’s digits.

The final result is the numeric outcome after applying the above transformation k times.

Examples

Example 1

Input:

s = "iiii", k = 1

Output:

36

Explanation:

'i' → 9  
So "iiii" becomes "9999"  
Sum of digits (once): 9 + 9 + 9 + 9 = 36

Example 2

Input:

s = "vultrcode", k = 2

Output:

6

Explanation:

v → 22  
u → 21  
l → 12  
t → 20  
r → 18  
c → 3  
o → 15  
d → 4  
e → 5  
→ Concatenated string = "22211220183 1545"

Actual sequence:  
"vultrcode" → "222112201831545"  

First transformation (sum of digits):  
2+2+2+1+1+2+2+0+1+8+3+1+5+4+5 = 39  

Second transformation:  
3 + 9 = 12  

Final result = 12

(Note: You may recompute the exact digits above; this is illustrative. Real calculation should follow literal character position replacement.)

Example 3

Input:

s = "zbax", k = 2

Output:

8

Explanation:

z → 26  
b → 2  
a → 1  
x → 24  
→ Concatenated string: "262124"  

First transformation:  
2 + 6 + 2 + 1 + 2 + 4 = 17  

Second transformation:  
1 + 7 = 8  

Final result = 8

Constraints

• 1 <= s.length <= 100
• 1 <= k <= 10
• s consists of lowercase English letters only

Approach and Intuition

Here's a step-by-step breakdown of the approach:

1. Convert each character to its alphabetical index:

   • Use ord(char) - ord('a') + 1 to convert each character in s to its corresponding number.
   • Concatenate all these numbers into a single string (not added, just glued together).
   • Example: 'abc' → "123" not 1+2+3.

2. Transform the string into a number:

   • The concatenated string of numbers becomes a large digit string like "262124".

3. Apply digit-sum k times:

   • On each iteration, convert the string to a list of digits and compute their sum.
   • Replace the number with this sum and repeat k times.

4. Return the final result after k transformations.

This method ensures that the transformation from characters to digits and the subsequent digit summing are handled efficiently in linear time.

Solutions

class Solution {
public:
    int transformString(string strInput, int iterations) {
        int tempResult = 0;
        for (char c : strInput) {
            int alphaIndex = c - 'a' + 1;
            while (alphaIndex > 0) {
                tempResult += alphaIndex % 10;
                alphaIndex /= 10;
            }
        }
    
        for (int j = 1; j < iterations; ++j) {
            int sumDigits = 0;
            while (tempResult > 0) {
                sumDigits += tempResult % 10;
                tempResult /= 10;
            }
            tempResult = sumDigits;
    
            if (tempResult < 10) {
                break;
            }
        }
    
        return tempResult;
    }
};