Watch 10 video solutions for Sum of Digits of String After Convert, a easy level problem involving String, Simulation. This walkthrough by ShashCode has 3,205 views views. Want to try solving it yourself? Practice on FleetCode or read the detailed text solution.
You are given a string s consisting of lowercase English letters, and an integer k. Your task is to convert the string into an integer by a special process, and then transform it by summing its digits repeatedly k times. More specifically, perform the following steps:
s into an integer by replacing each letter with its position in the alphabet (i.e. replace 'a' with 1, 'b' with 2, ..., 'z' with 26).k times in total.For example, if s = "zbax" and k = 2, then the resulting integer would be 8 by the following operations:
"zbax" ➝ "(26)(2)(1)(24)" ➝ "262124" ➝ 262124262124 ➝ 2 + 6 + 2 + 1 + 2 + 4 ➝ 1717 ➝ 1 + 7 ➝ 8Return the resulting integer after performing the operations described above.
Example 1:
Input: s = "iiii", k = 1
Output: 36
Explanation:
The operations are as follows:
- Convert: "iiii" ➝ "(9)(9)(9)(9)" ➝ "9999" ➝ 9999
- Transform #1: 9999 ➝ 9 + 9 + 9 + 9 ➝ 36
Thus the resulting integer is 36.
Example 2:
Input: s = "leetcode", k = 2
Output: 6
Explanation:
The operations are as follows:
- Convert: "leetcode" ➝ "(12)(5)(5)(20)(3)(15)(4)(5)" ➝ "12552031545" ➝ 12552031545
- Transform #1: 12552031545 ➝ 1 + 2 + 5 + 5 + 2 + 0 + 3 + 1 + 5 + 4 + 5 ➝ 33
- Transform #2: 33 ➝ 3 + 3 ➝ 6
Thus the resulting integer is 6.
Example 3:
Input: s = "zbax", k = 2
Output: 8
Constraints:
1 <= s.length <= 1001 <= k <= 10s consists of lowercase English letters.Problem Overview: You receive a lowercase string s and an integer k. Each character is converted to its alphabet position (a = 1, b = 2, … z = 26) and concatenated into a numeric string. Then you repeat the digit-sum operation k times. The goal is to return the final integer after these transformations.
Approach 1: Map and Sum Technique (O(n + k * d) time, O(n) space)
This approach directly simulates the process described in the problem. First iterate through the string and convert each character using value = c - 'a' + 1. Append these numbers to build the transformed numeric string. After that, perform the digit-sum operation k times by iterating through the digits and computing their total. The main work comes from scanning the original string once and then processing the digit string repeatedly. This approach is straightforward and mirrors the problem statement, making it easy to implement when practicing string manipulation and simulation.
Approach 2: Direct Digit Sum Accumulation (O(n + k) time, O(1) space)
The key observation: you do not need to build the intermediate numeric string. When converting characters to alphabet positions, immediately split multi-digit values and add their digits to a running sum. For example, z = 26 contributes 2 + 6. This gives the result of the first transformation directly as a digit sum. After that, repeat the digit-sum operation k - 1 times on the resulting number until all iterations are completed. Since only a few digits are processed after the first step, the remaining operations are very small. This optimized simulation avoids storing large strings and keeps space constant while still using simple iteration.
Recommended for interviews: Direct Digit Sum Accumulation is the approach most interviewers expect. It shows you spotted the optimization that eliminates the intermediate string and reduces memory usage. Implementing the basic Map and Sum simulation first demonstrates understanding of the transformation, but the optimized approach proves stronger reasoning with digit manipulation and efficient string processing.
| Approach | Time | Space | When to Use |
|---|---|---|---|
| Map and Sum Technique | O(n + k * d) | O(n) | When implementing the problem exactly as described or when learning basic string-to-number transformations. |
| Direct Digit Sum Accumulation | O(n + k) | O(1) | Best for interviews and production code where avoiding extra string storage improves efficiency. |