Essentially, the normal of the vertex is the average of the adjacent faces' (triangles) normals.

In pseudocode:

for each face adjacent to vertex[n]
    sum = sum + face.normal
normal[n] = NORMALIZE(sum / COUNT(adjacent faces))

Repeat for each vertex.

Essentially, the normal of the vertex would be the average of the adjacent faces' (triangles) normals.

In pseudocode:

for each face adjacent to vertex[n]
    sum = sum + face.normal
normal[n] = NORMALIZE(sum / COUNT(adjacent faces))

Repeat for each vertex.