Abstract: Lexical analyzers are important components of compilers and interpreters for coding languages such as C, Java, and C++. It acts as the first stage of the compiler, transforming the source ...

Given a digit string, return all possible letter combinations that the number could represent. A mapping of digit to letters (just like on the telephone buttons) is given below.

Given a binary tree, return the values of its boundary in anti-clockwise direction starting from root. Boundary includes left boundary, leaves, and right boundary in order without duplicate nodes.