Monotonic Stack

function dailyTemperatures(temperatures: number[]): number[] {
  const res = new Array<number>(temperatures.length).fill(0);
  const stack: number[] = []; // indices, temps decreasing down the stack
  for (let i = 0; i < temperatures.length; i++) {
    while (stack.length && temperatures[i] > temperatures[stack[stack.length - 1]]) {
      const j = stack.pop()!;
      res[j] = i - j;
    }
    stack.push(i);
  }
  return res;
}

def daily_temperatures(temperatures)
  res = Array.new(temperatures.length, 0)
  stack = [] # indices, temperatures decreasing down the stack
  temperatures.each_with_index do |temp, i|
    while !stack.empty? && temp > temperatures[stack.last]
      j = stack.pop
      res[j] = i - j
    end
    stack.push(i)
  end
  res
end