Pretty mechanical once you see the pattern.
Clarify that the sequence starts with '1' as the first term, then iteratively generate each next term by scanning the previous string and appending the count and digit for each run of identical digits. Implement this with a simple loop that builds the next term from the current one, repeating n-1 times, and discuss time/space complexity.
Pro tip: Mention that the sequence grows exponentially (about 1.3^n), so for large n you should consider memory limits and possibly generate the nth term without storing all previous terms. Also, clarify edge cases like n=1 and n=0 upfront.
Confirm that the first term is '1' and that we need to return the nth term. Ask about constraints (e.g., maximum n) and handle edge cases like n=0 or n=1.
Start with the current term as '1'. For each step from 2 to n, scan the current term, count consecutive identical digits, and build the next term by appending count followed by digit.
Use a loop with a pointer to traverse the string, tracking the current digit and its count. When the digit changes, append the count and digit to a result list, then reset for the new digit.
Discuss that time complexity is O(total length of all terms up to n), which is exponential in n. Mention that space can be optimized to O(length of current term) by only keeping the previous term.
Walk through small n (e.g., n=1 -> '1', n=2 -> '11', n=3 -> '21', n=4 -> '1211') to verify correctness and edge cases.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
This one tripped me up more than I expected.
First, clarify the problem and edge cases, then explain the key insight: a string can be rearranged into a palindrome if at most one character has an odd frequency. Use a bitmask to represent the parity of character counts, and for each string, count substrings with at most one set bit using a hash map of prefix masks.
Pro tip: Mention that the bitmask approach works because there are only 26 lowercase letters, so the mask fits in an integer. Also, note that the solution runs in O(n) per string, which is optimal for this problem.
Confirm that substrings are contiguous, non-empty, and that rearrangement means any permutation of the characters. Ask about input size and constraints to determine the expected complexity.
Explain that a string can be rearranged into a palindrome if and only if at most one character has an odd count. This is the key property to check for each substring.
Represent the parity of character counts as a 26-bit integer. Compute prefix masks for each position, where the mask at index i represents the parity of characters in the substring from start to i.
For each prefix mask, count how many previous prefix masks differ by at most one bit (i.e., have Hamming distance 0 or 1). Use a hash map to store frequencies of prefix masks and iterate through the string.
State that the time complexity is O(n) per string and space O(n) for the hash map. Discuss edge cases like empty strings, single characters, and strings with all identical characters.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Use backtracking to explore all possible digit assignments for '?' positions, pruning branches where the current sum plus the minimum possible remaining sum exceeds the target or the maximum possible remaining sum falls short. For each complete assignment, check if the total sum equals the target and add the resulting string to the output. This ensures efficiency by avoiding unnecessary recursion.
Pro tip: Precompute the sum of fixed digits and the number of '?' positions to quickly determine if a solution is possible (target must be between fixedSum and fixedSum + 8*numQuestionMarks). Also, during backtracking, prune early by checking if the remaining sum can be achieved with the remaining '?' positions.
Calculate the sum of fixed digits and count the number of '?' positions. Check if the target sum is within the feasible range [fixedSum, fixedSum + 8 * numQuestionMarks]; if not, return an empty list immediately.
Convert the string to a list of characters for easy modification. Define a recursive function that takes the current index, the current sum, and the list of characters.
At each '?' position, iterate digits 0-8. For each digit, update the sum and recurse. Prune if the current sum plus the minimum possible sum from remaining '?' (0 each) exceeds the target, or if the current sum plus the maximum possible sum from remaining '?' (8 each) is less than the target.
When all positions are processed, if the current sum equals the target, convert the character list back to a string and add it to the results list.
After the recursion completes, return the list of valid completed strings.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.