Well, I am glad to share my insights with you. The problem asks us to count the number of "Right Triangles"(you must know what the "RT" is), the Picture is described by '.' '*', we need to count the triangles consisted of '*'. The first Example is , the ans equals the number of '*'(note that one '*' can make a triangle itself); And the example below has the two triangles consisted of 3 '*' extra. (ignore my pool English)