You are given a string tiles containing uppercase English letters, where each letter represents a tile. You need to return the number of distinct non-empty sequences of letters that can be formed by selecting tiles from the string. A sequence can be formed by selecting tiles in any order and any number of times, but no tile can be used more than once in a sequence.
In a gold mine grid of size m x n, each cell contains an integer representing the amount of gold in that cell. A cell with 0 means no gold is present. The task is to find the maximum amount of gold that can be collected by following certain movement rules: You can move one step in any of the four directions (left, right, up, down), but you cannot visit the same cell twice, and you cannot move to a cell that contains 0 gold.
Given two integers n and start, return a permutation of the integers from 0 to 2^n - 1 such that: p[0] = start, each pair of adjacent elements differ by exactly one bit in their binary representation, and p[0] and p[2^n - 1] also differ by exactly one bit.
You are given an array of strings arr. Return the maximum possible length of a string s formed by concatenating a subsequence of arr such that all characters in s are unique.
Design a CombinationIterator class that generates all combinations of a specified length from a sorted string of distinct lowercase English letters, and allows iterating through them in lexicographical order.
A happy string consists of only the letters ‘a’, ‘b’, and ‘c’, and no two consecutive characters in the string are the same. Given two integers n and k, return the kth happy string of length n sorted in lexicographical order, or an empty string if there are fewer than k happy strings of length n.