Given a sorted array ’nums’ in non-decreasing order, find the starting and ending index of a target value. If the target is not found, return [-1, -1]. Your solution must run in O(log n) time complexity.
Determine if a 9x9 Sudoku board is valid. A valid Sudoku board follows three rules: Each row, column, and 3x3 sub-box must contain the digits 1-9 without repetition. Only the filled cells need to be validated.
The Count-and-Say sequence starts with ‘1’. Each subsequent term is generated by describing the previous term in terms of the count of consecutive digits. Given a positive integer n, return the nth term of the Count-and-Say sequence.
Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order. The same number can be chosen from candidates an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different.
You are given a collection of distinct candidate numbers and a target number. Your task is to find all unique combinations of the candidate numbers that sum up to the target. Each number in the candidates list may only be used once in each combination. The solution should exclude duplicate combinations.
Given two non-negative integers represented as strings, return their product as a string. You must not use any built-in BigInteger library or convert the inputs to integers directly.
