Leetcode 2243: Calculate Digit Sum of a String
You are given a string of digits,
s, and an integer k. Your task is to repeatedly process the string until its length becomes less than or equal to k. In each step, divide the string into consecutive groups of size k. If the last group is smaller than k, process it as is. For each group, calculate the sum of its digits, convert the result back to a string, and merge all groups to form a new string. Repeat the process until the string length is ≤ k, and return the final string.Problem
Approach
Steps
Complexity
Input: The input consists of a string `s` and an integer `k`.
Example: For example, `s = "987654"` and `k = 2`.
Constraints:
• 1 <= s.length <= 100
• 2 <= k <= 100
• The string `s` consists of digits only.
Output: The function returns the final processed string after applying the digit-sum transformation until the length of `s` becomes less than or equal to `k`.
Example: If `s = "987654"` and `k = 2`, the output will be `"3011"`.
Constraints:
• The output string will have a length of at most `k`.
Goal: Transform the string `s` iteratively by dividing it into groups of size `k`, summing the digits of each group, and merging the results until its length is less than or equal to `k`.
Steps:
• While the length of `s` is greater than `k`, perform the following:
• 1. Divide `s` into groups of size `k`.
• 2. For each group, calculate the sum of its digits.
• 3. Replace the group with the sum as a string.
• 4. Merge all groups to form the new string `s`.
• Return `s` when its length is less than or equal to `k`.
Goal: Ensure the transformation respects input constraints.
Steps:
• The input string `s` must consist only of numeric digits.
• The integer `k` must be at least 2 and no larger than the length of `s`.
Assumptions:
• The input string `s` is non-empty.
• The value of `k` is always valid.
• Input: Input: `s = "987654321", k = 3`
• Explanation: 1. Divide `s` into groups: `"987"`, `"654"`, `"321"`.
2. Sum digits in each group: `9+8+7=24`, `6+5+4=15`, `3+2+1=6`.
3. Merge results: `"24156"`. Repeat for `"24156"`.
4. Divide: `"241"`, `"56"`. Sum: `2+4+1=7`, `5+6=11`. Merge: `"711"`. Stop as length ≤ `k`.
• Input: Input: `s = "1234", k = 2`
• Explanation: 1. Divide: `"12"`, `"34"`. Sum: `1+2=3`, `3+4=7`. Merge: `"37"`. Stop as length ≤ `k`.
Approach: Iteratively process the string until its length is less than or equal to `k` by dividing into groups, summing digits, and merging results.
Observations:
• Each group can be processed independently.
• The transformation involves simple arithmetic operations.
• Use a loop to handle iterations.
• Handle groups efficiently using slicing or indexing.
Steps:
• Initialize a loop to process the string while its length exceeds `k`.
• Slice the string into groups of size `k`.
• Calculate the sum of digits for each group.
• Convert the sums to strings and concatenate them.
• Update the string and repeat until its length is less than or equal to `k`.
Empty Inputs:
• Not applicable as `s` is guaranteed to be non-empty.
Large Inputs:
• Ensure the implementation handles strings close to the maximum length of 100 efficiently.
Special Values:
• `k` equals the length of `s`: return `s` immediately.
• `s` contains only zeros: ensure the sum correctly results in zeros.
Constraints:
• Ensure all calculations respect numeric constraints for digits.
string digitSum(string s, int k) {
while(s.size() > k) {
string s1;
for (int i = 0; i < s.size(); i += k)
s1 += to_string(accumulate(begin(s) + i, begin(s) + min((int)s.size(), i + k), 0,
[](int n, char ch){ return n + ch - '0'; }));
swap(s1, s);
}
return s;
}
1 : Function Definition
string digitSum(string s, int k) {
This line defines the function 'digitSum', which takes a string 's' and an integer 'k' as input.
2 : Loop Condition
while(s.size() > k) {
The while loop continues as long as the size of the string 's' is greater than 'k'.
3 : Variable Initialization
string s1;
A temporary string 's1' is initialized to store the sum of digits in each segment.
4 : For Loop
for (int i = 0; i < s.size(); i += k)
This for loop iterates over the string 's' in steps of 'k', processing 'k' digits at a time.
5 : Accumulation
s1 += to_string(accumulate(begin(s) + i, begin(s) + min((int)s.size(), i + k), 0,
This line accumulates the sum of the 'k' digits starting from index 'i'. The 'accumulate' function adds each digit's numeric value to the total.
6 : Lambda Function
[](int n, char ch){ return n + ch - '0'; }));
The lambda function inside 'accumulate' converts each character 'ch' to its corresponding integer value by subtracting '0'.
7 : Swap Operation
swap(s1, s);
After each iteration, the new string 's1' is swapped with 's', so the process continues on the updated string.
8 : Return Statement
return s;
Once the string size is less than or equal to 'k', the function returns the final string 's'.
Best Case: O(n)
Average Case: O(n log n)
Worst Case: O(n log n)
Description: Each iteration processes the string in linear time, and the length of `s` reduces logarithmically over iterations.
Best Case: O(1)
Worst Case: O(n)
Description: In-place modifications reduce space requirements; however, slicing may require extra memory proportional to the input size.
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