HDU 2424-Gary's Calculator（表達式計算+大數）

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 Description Gary 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.

Input There 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 should 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.

Output For 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

3
100 + 600
3
20 * 4
2
+ 500
5
20 + 300 * 20

Sample Output

Case 1: 700
Case 2: 80
Case 3: Invalid Expression!
Case 4: 6020給出一個只含+和*的表達式，求其值。由於數據太大，使用BigInteger 。 可以采用棧來計算表達式，先算*，後算+;
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);
		}
	}
}