Stacks and Queues

Stack Push/Pop

Push values onto a stack and pop them back in last-in, first-out order.

Algorithm

The replay uses the same three values in every language, so this R DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.R 
render <- function(values) {
  paste(values, collapse = " -> ")
}

stack <- c()
for (value in c(10, 20, 30)) stack <- c(stack, value)
popped <- c()
while (length(stack) > 0) {
  popped <- c(popped, stack[length(stack)])
  stack <- stack[-length(stack)]
}
cat(render(popped), "\n", sep = "")

Complexity

  • Time: O(1) per push/pop
  • Space: O(n)

Implementation notes

  • Keep the explicit stack/queue operations. Library shortcuts that only produce the final list hide the data-structure behavior this lesson is meant to replay.
  • The final output uses a deterministic a -> b -> c format for cross-language comparison.
top The top is the most recently pushed value.
LIFO A stack removes values in last-in, first-out order.