public static int solution_Stonewall(int[] H)
{
// observations:
// if a same height wall is adjacent, a single block can be used
// if the height of the wall is unique, a single block must be used to represent that unique height
// looks like a stack should be used to store the height of each block, then trying to build on it
// when the stack cannot be used to construct the next wall, should be discarded
Stack <long> stack = new Stack <long>();
int blocks = 1;
stack.Push(H[0]);
//scan from left to right
for (int i = 1; i < H.Length; i++)
{
long stackheight = stack.Sum();
if (H[i] < stackheight)
{
if (stack.Count == 1)
{
blocks++;
stack.Pop();
stack.Push(H[i]);
continue;
}
bool blockheightexists = false;
int sum = 0;
foreach (int block in stack.Reverse())
{
sum += block;
//height already exist
if (sum == H[i])
{
blockheightexists = true;
while (H[i] < stack.Sum())
{
stack.Pop();
}
break;
}
}
if (!blockheightexists)
{
blocks++;
while (H[i] < stack.Sum())
{
stack.Pop();
}
stack.Push(H[i] - stack.Sum());
}
}
else if (H[i] > stackheight)
{
blocks++;
stack.Push(H[i] - stack.Sum());
}
}
return(blocks);
}