A Study to Compare the Administration of Pembrolizumab After Surgery Versus Administration Both Before and After Surgery for High-Risk Melanoma

Inclusion Criteria: * STEP 1 REGISTRATION (RANDOMIZATION): Patients must have clinically detectable stage III (clinically detectable N1b, N1c, N2b, N2c, N3b and N3c) or stage IV resectable melanoma. Patients with melanoma of mucosal or acral origin are eligible. Patients with melanoma of uveal origin are not eligible. Patients with a history of brain metastases are not eligible. Clinically detectable is defined as disease that is apparent and measurable via physical examination or radiographic imaging. * STEP 1 REGISTRATION (RANDOMIZATION): Patients are eligible for this trial either at initial presentation of their melanoma or at the time of the first detected nodal, satellite/in-transit, distant metastases, or recurrent disease in prior lymphadenectomy basin or distant site. Nodal, satellite/in-transit metastasis, distant metastases or disease in a prior complete lymphadenectomy basin must have been confirmed histologically by hematoxylin (H) \& eosin (E) stained slides. * STEP 1 REGISTRATION (RANDOMIZATION): Patients with multiple regional nodal basin involvement are eligible. Gross or microscopic extracapsular nodal extension is permitted. * STEP 1 REGISTRATION (RANDOMIZATION): Patients must have histologically proven stage IIIB or higher. This would entail pathologic confirmation beyond the primary or initial diagnosis of melanoma involving fine needle aspiration cytology or biopsy confirmation of any N-category or M-category resectable site. * STEP 1 REGISTRATION (RAND…