Leetcode 1625. Lexicographically Smallest String After Applying Operations  Summary and Q&A
TL;DR
Explore methods to find the smallest string through two operations on a numeric string.
Key Insights
 π» The allowed operations on the string yield multiple unique configurations, potentially reaching up to 1000 distinct strings.
 πͺ Applying modulo 10 during the add operation prevents values from exceeding the digit bounds and simplifies numeric transitions.
 π¨βπ¬ Both depthfirst search and breadthfirst search methods can be utilized to exhaustively explore string modifications while tracking visited states to optimize pathfinding towards the result.
 π§ Understanding the challenges of achieving the lexicographically smallest string requires strategic considerations of digit impacts and position relevance in the string.
 π The algorithm's performance hinges on efficiently managing the exploration of sequences and minimizing repeated calculations through state checks.
 π§ Careful selection of odd positions during operations becomes crucial to maximizing the resulting string's minimization potential.
 π¨βπ¬ Limiting computational complexity to O(10^5) reflects a balance between exhaustive search and practical execution time in competitive programming environments.
Transcript
hey there everyone welcome back to lead coding in this video we are going to solve lexicographically smallest string after applying operations the problem statement is we are given a string s of even length which contains digits from 0 to 9 and we are given two integers a and b we can perform either of the two operations in any sequence any number ... Read More
Questions & Answers
Q: What are the two operations that can be performed on the string?
The two operations are: first, adding a given integer to the digits located at the odd positions in the string, applying modulo 10 to the result; second, rotating the string to the right by a specified number of positions.
Q: How does the add operation work in the context of the problem?
The add operation involves iterating through the string and adding a specified integer, a
, to each digit at odd indices. After adding, we take the result modulo 10. For example, if a digit at an odd position is 6 and a
is 5, the result becomes 1 after taking modulo 10.
Q: Why is it important to track visited strings?
Tracking visited strings helps prevent redundant operations and ensures that the algorithm doesnβt get stuck in cycles, which optimizes performance and reduces time complexity. It allows the exploration of unique string configurations and helps in converging towards the lexicographically smallest result.
Q: What is the complexity of the approach used in the solution?
The time complexity of the approach, owing to the unique string possibilities and each operation's execution time, is O(10^5), while the space complexity is O(10^4) due to storing the seen strings in a visited set.
Q: How can the solution be implemented using breadthfirst search (BFS)?
BFS can be implemented by utilizing a queue to explore each string state. Starting from the initial string, we can enqueue the results from both operations, ensuring each visited state is checked to avoid reprocessing and helping maintain the minimal string discovery.
Q: What assumptions are made about the inputs for string s
?
The string s
is assumed to have an even length and consists only of digits from 0 to 9. The operations allowed also depend on integer parameters a
and b
, which should fall within defined limits mentioned in the problem statement.
Q: How do the operations affect the string's lexicographical order?
The operations can significantly alter the string's arrangement and individual digit values, making it either smaller or larger in lexicographic order. By selectively applying these operations, it's possible to strategically position smaller digits at the front of the string, lowering its overall value.
Q: Why was brute force accompanied by optimizations in the proposed solution?
While the brute force method involves checking all possible configurations, optimizations such as using sets to track visited strings and limiting operations based on the maximum unique states allowed help reduce computational overhead and quickly converge on a solution, making it more efficient.
Summary & Key Takeaways

The problem involves a string containing digits and allows two operations: adding an integer to odd positions and rotating the string. The goal is to find the lexicographically smallest string possible using these operations.

With a maximum string length of 100 and by performing a combination of operations, there are limited unique strings that can be generated, estimated at 1000 unique combinations in the worstcase scenario.

The presented solution uses depthfirst search (DFS) or breadthfirst search (BFS) to explore all possible string states while maintaining a record of visited strings to avoid repetitions and optimize the search for the minimal string.