Implement Min Heap

JavaScript has no built-in heap, so interviews often ask you to write one. Implement a binary min-heap of integers.

Implement the MinHeap class:

• MinHeap() creates an empty heap.
• push(val) inserts val.
• pop() removes and returns the smallest value, or -1 if the heap is empty.
• peek() returns the smallest value without removing it, or -1 if the heap is empty.
• size() returns the number of values stored.

All stored values are non-negative, so -1 is never a real value. push and pop must run in O(log n); peek and size in O(1). Duplicates are allowed.