Short Problem Definition:
John has discovered various rocks. Each rock is composed of various elements, and each element is represented by a lower-case Latin letter from ‘a’ to ‘z’. An element can be present multiple times in a rock. An element is called a gem-element if it occurs at least once in each of the rocks.
Given the list of N rocks with their compositions, display the number of gem-elements that exist in those rocks.
time complexity is O(N*T)
space complexity is O(N)
We count the number of elements that occur in all characters sets. Just use set intersection.