What is the efficient algorithm to implement Stack? - algorithm

Question

I have come across a problem. I need to implement stack with push and pop operations.
Input
The first line of the input file contains a single integer number N (1 <= N <= 10^6) – the number of test cases.
Next N lines tells about operations. + means push. - means pop. I need to print popped element.
Example
Input Output
6
+ 1 10
+ 10 1234
-
+ 2
+ 1234
-
I have written following code
public class Main {
public static void main(String[] args) throws FileNotFoundException {
Scanner sc = new Scanner(new File("stack.in"));
PrintWriter pw = new PrintWriter(new File("stack.out"));
int n=sc.nextInt();
int[] stack = new int[n]; int i=0;
while(n-->0) {
String s = sc.next();
if(s.equals("+")) {
stack[i++]=sc.nextInt();
} else {
pw.println(stack[--i]);
}
}
sc.close(); pw.close();
}
}
This program is giving me Time Limit Exceeded.
Please suggest me an efficient algorithm to solve this.
For each input file:
Time limit: 2 seconds
Memory limit: 256 megabytes

Answer 1

A rule of thumb: if you're solving a competitive programming style problem and the input is large (say, 10^5 numbers or more), the Scanner is too slow.
You can use a StringTokenizer on top of a BufferedReader to speed up the input.
It can look like this:
class FastScanner {
private StringTokenizer tokenizer;
private BufferedReader reader;
public FastScanner(InputStream inputStream) {
reader = new BufferedReader(new InputStreamReader(inputStream));
}
public String next() {
while (tokenizer == null || !tokenizer.hasMoreTokens()) {
String line;
try {
line = reader.readLine();
} catch (IOException e) {
throw new RuntimeException(e);
}
if (line == null)
return null;
tokenizer = new StringTokenizer(line);
}
return tokenizer.nextToken();
}
public int nextInt() {
return Integer.parseInt(next());
}
}