Trapping Water - Stack Approach

Use a monotonic decreasing stack. When you encounter a bar taller than the stack top:

$1.$ Pop the top (the popped element forms a valley)

$2.$ If stack is now empty, no water can be trapped

$3.$ Otherwise, calculate water between current bar, popped bar, and new top

The water at this layer has width $(i - stack.top() - 1)$ and height $\min(current, newTop) - popped$. Accumulate water as you pop.