579. Find Cumulative Salary of an Employee

{"headers":{"Employee":["id","month","salary"]},"rows":{"Employee":[[1,1,20],[2,1,20],[1,2,30],[2,2,30],[3,2,40],[1,3,40],[3,3,60],[1,4,60],[3,4,70],[1,7,90],[1,8,90]]}}