in c++

Write a function int howmuchwater(unsigned int a[], int n)

where a[] is a 1D array where a[i] reports the number of blocks stacked atop square i n is the number of elements in a[] and returns the cubic feet of trapped water from blocks described in a[] if n>0 and a[] is non-null, otherwise returns -1.

Examples:

a)

 b b b b b b b b b ++++++++++ 7

b)

 b b bb b b bb bbbbbbb +++++++++++++ 6

c)

+++ 0

d)

 b +++ 0

e) -1