task scheduler

Given a list of tasks with execution time of each task and its dependecies.
e.g. 
A, 2 <== task A needs 2 timeunits to finish
B, 3 <== taks B needs 3 timeunits to finish
C, 5
D, 3

B, C can run only after A finishes. D can run only after B finished. 
A -> B,C
B -> D

The task scheduler should output the followings: 
["A,2", "B,C,3", "C,D,2", "D,1"]

which means:
step1: A, 2.  <== A will start first, finish in 2 timeunits
step2: B, C, 3 <== B will finish in step 2, but C won't
step3: C, D, 2 <== C will finish in step 3, but D won't
step4: D, 1

Implement the following method:
List<String> scheduler(Map<String, Integer> taskExecutionTime, Map<String, String> dependencies)