The "Check If String Is Transformable With Substring Sort Operations" interview question is a deep dive into string manipulation. You are given two strings, s and t. You can perform an operation where you choose a substring of s and sort it. You can do this any number of times. The goal is to determine if you can transform s into t. While sorting sounds broad, the key is realizing that you can move a character to the left as long as no smaller characters block its path.
Google uses this "Hard" level problem to test a candidate's ability to simplify complex operations. "Sorting any substring" can be reduced to a much simpler primitive: "Can I move a digit past another digit ?". This problem evaluates your Greedy thinking and your ability to use data structures like queues to track the positions of characters efficiently.
This problem uses a Greedy approach combined with a Position Tracking pattern. Since you can only sort a substring, you can only move a digit to the left if all digits to its left are greater than or equal to it. You maintain 10 queues (one for each digit 0-9) storing the indices of those digits in s. For each digit in t, you check if its next available index in s is "unblocked" by any smaller digits.
t, it must pass 4 and 5. Since 1 is smaller than 4 and 5, it can pass.
The check is essentially: for a digit , are there any indices of digits that appear before the current index of in ?A major mistake is trying to actually simulate the sorting, which is impossible due to the infinite number of combinations. Another error is not checking if the character counts in s and t are identical first. Candidates also often forget that a digit only needs to be "clear" of digits smaller than itself to move left via a sort operation.
When an operation like "sort any substring" is given, try to find the "minimal" move it allows. Often, these complex rules boil down to simple "can A pass B?" checks. Practice problems involving character positioning and blocking to master this pattern.