A message containing letters from A-Z can be encoded into numbers using the mapping. To decode an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways).
A stack is an abstract data type that stores an ordered collection of items which follow first in last out (FILO) or last in first out (LIFO) principle; with two principal operations: push, which adds an item to the stack, and pop, which removes the most recently added item.
Dijkstra's algorithm is an algorithm for finding the shortest paths between nodes in a graph, which may represent, for example, road networks. It was conceived by computer scientist Edsger W. Dijkstra in 1956 and published three years later. The algorithm exists in many variants.
Given a string, find the longest palindromic substring. We going to iterate each character in the string; and within that character, we expand and look for the longest palindrome. As we keep iterating, we only keep the longest palindrome.