HDU 2424-gary's calculator (expression calculation + large number)

Gary's Calculator Time Limit: 5000/1000 MS (Java/others) memory limit: 32768/32768 K (Java/Others)
Total submission (s): 950 accepted submission (s): 209


Problem descriptiongary has finally decided to find a calculator to avoid making simple calculational mistakes in his math exam. unable to find a suitable calculator in the market with enough precision, Gary has designed a high-precision calculator himself. can you help him to write the necessary program that will make his design possible?
For simplicity, you only need to consider two kinds of calculations in your program: Addition and multiplication. it is guaranteed that all input numbers to the calculator are non-negative and without leading zeroes.

Inputthere are multiple test cases in the input file. each test case starts with one positive integer n (n <20), followed by a line containing N strings, describing the expression which Gary's calculator showould evaluate. each of the N strings might be a string representing a non-negative integer, a "*", or a "+ ". no integer in the input will exceed 109.
Input ends with end-of-file.

Outputfor each test case, please output one single INTEGER (with no leading zeros), the answer to Gary's expression. If Gary's expression is invalid, output "invalid expression! "Instead. Please use the format indicated in the sample output.

Sample Input

3100 + 600320 * 42+ 500520 + 300 * 20

Sample output

Case 1: 700Case 2: 80Case 3: Invalid Expression!Case 4: 6020给出一个只含+和*的表达式，求其值。由于数据太大，使用BigInteger 。 可以采用栈来计算表达式，先算*，后算+;
import java.io.*;import java.util.*;import java.math.*;public class Main {public static void main(String[] args) {Scanner in = new Scanner(System.in);int n, cas = 1;while (in.hasNext()) {n = in.nextInt();String[] s = new String[n];for (int i = 0; i < n; i++)s[i] = in.next();int flag = 1;if (n % 2 == 0)flag = 0;for (int i = 0; i < n; i++) {if ((i % 2 == 1 && s[i].charAt(0) != '+' && s[i].charAt(0) != '*')|| (i % 2 == 0 && (s[i].charAt(0) == '+' || s[i].charAt(0) == '*')))flag = 0;}System.out.print("Case " + cas + ": ");cas++;if (flag == 0 || n == 0) {System.out.println("Invalid Expression!");continue;}Stack stack = new Stack();for (int i = 0; i < n; i++) {int tag = 0;if (s[i].charAt(0) != '+' && s[i].charAt(0) != '*') {if (stack.size() != 0) {String t = stack.peek().toString();if (t.charAt(0) == '*') {stack.pop();if (stack.size() != 0) {String tt = stack.pop().toString();BigInteger a = new BigInteger(s[i].toString());BigInteger b = new BigInteger(tt.toString());stack.push(a.multiply(b).toString());tag = 1;}}}}if (tag == 0)stack.push(s[i]);}BigInteger res = BigInteger.ZERO;while (stack.size() != 0) {String x = stack.pop().toString();if (x.charAt(0) != '+') {res = res.add(new BigInteger(x));}}System.out.println(res);}}}

HDU 2424-gary's calculator (expression calculation + large number)