Monotonic Stack Visualization

Array: $[2, 1, 2, 4, 3]$. Process left to right, maintain decreasing stack. $i=0$, val=$2$: stack empty, push $2$. Stack: $[2]$

$i=1$, val=$1$: $1 < 2$, push $1$.

Stack: $[2, 1]$

$i=2$, val=$2$: $2 > 1$, pop $1$ (answer for index $1$ is $2$), push $2$. Stack: $[2, 2]$

$i=3$, val=$4$: $4 > 2$, pop both $2$s (answers are $4$), push $4$.

Stack: $[4]$

$i=4$, val=$3$: $3 < 4$, push $3$. Stack: $[4, 3]$

Remaining elements ($4$ and $3$) have no next greater.