Describe how you could use a single array to implement three stacks.

You should implement push(stackNum, value)、pop(stackNum)、isEmpty(stackNum)、peek(stackNum) methods. stackNum is the index of the stack. value is the value that pushed to the stack.

The constructor requires a stackSize parameter, which represents the size of each stack.

Example1:

 Input: 

["TripleInOne", "push", "push", "pop", "pop", "pop", "isEmpty"]

[[1], [0, 1], [0, 2], [0], [0], [0], [0]]

 Output: 

[null, null, null, 1, -1, -1, true]

Explanation: When the stack is empty, `pop, peek` return -1. When the stack is full, `push` does nothing.

Example2:

 Input: 

["TripleInOne", "push", "push", "push", "pop", "pop", "pop", "peek"]

[[2], [0, 1], [0, 2], [0, 3], [0], [0], [0], [0]]

 Output: 

[null, null, null, null, 2, 1, -1, -1]