Image of the Question
All Testcases Passed ✔
Solution
#!/bin/python3
import math
import os
// ... rest of solution available after purchaseExplanation
```
To solve the problem of encoding a string by collapsing consecutive occurrences of the same character into a count followed by that character (often called run-length encoding), you need to think systematically about how to process the string and track sequences of repeated characters.
Step 1: Understand the input and output
You are given a string consisting of alphanumeric characters. Your goal is to transform it into a compressed format where consecutive runs of the same character are replaced by the count of that run followed by the character itself. For example, a sequence of five 'G's becomes "5G". Single occurrences are also encoded as "1X" where X is the character.
Step 2: Handle edge cases
If the input string is empty, the output should be an empty string. It is important to check for this case first to avoid errors when processing the string.
Step 3: Initialize variables to track the current run
Set up a variable to keep track of the current character you are counting and another variable to keep track of how many times it has appeared consecutively. Initially, these would be set to the first character of the string and a count of 1 respectively.
Step 4: Iterate through the string
Traverse the string character by character starting from the second character. For each character, compare it to the current character being tracked.
- If it is the same, increment the count.
- If it is different, append the current count and the character being tracked to the output string and reset the tracking variables for the new character with a count of 1.
This way, whenever a new character is encountered, the previous run’s information is saved.
Step 5: Handle the final run after iteration
After the iteration completes, the last run will not yet be appended to the output because the loop ends before this happens. Therefore, append the count and character of the last tracked run to the output.
Step 6: Construct the output string efficiently
As you track runs, you will want to build the resulting encoded string. Consider using a list or similar structure to collect pieces of the encoded string, and join them at the end. This avoids inefficient string concatenation inside loops.
Step 7: Consider input constraints
The input can contain alphanumeric characters, meaning letters (both uppercase and lowercase) and digits. The approach remains the same for all characters since you are only looking at consecutive identical characters regardless of type.
Step 8: Confirm correctness with examples
- For the input "GGGGGrrrrrrrrrrrrttt":
* Start counting 'G's → 5G
* Then count 'r's → 14r
* Then count 't's → 3t
- For a string like "AABBCC":
* 2A2B2C
- For a string with single characters only, like "ABC":
* 1A1B1C
Step 9: Complexity considerations
The solution requires a single pass through the string, resulting in O(n) time complexity, which is optimal for this problem since you must inspect each character. The space complexity depends on the output string size, which in the worst case can be roughly twice the input size (every character different, thus outputting "1X" pairs).
Summary:
1. Check for empty string, return empty output if so.
2. Initialize count and current character with the first character.
3. Iterate through the string starting from the second character.
4. Increment count if the same character; otherwise, append count and character to output and reset count.
5. After iteration, append the final run count and character.
6. Return the constructed output string.
By following this approach, you will create an efficient and correct run-length encoding of the input string.
```