Backtracking

Given an array of distinct positive integers candidates and a target integer target, return all unique combinations of candidates whose sum equals target. You may use the same number any number of times.

The N-Queens puzzle places n queens on an n x n chessboard so that no two queens share a row, column, or diagonal. Given n, return all distinct solutions; each solution is the board drawn with 'Q' marking a queen and '.' marking an empty square.

Design an algorithm to solve a Sudoku puzzle, which is a 9x9 grid filled with numbers from 1-9, where each row, column, and 3x3 sub-grid must contain each number only once.