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import static org.junit.Assert.assertSame;
public class EvaluateReversePolishNotation {
public int evalRPN(String[] tokens) {
if (tokens == null || tokens.length == 0) {
return 0;
}
Stack<Integer> stack = new Stack<Integer>();
for (String s : tokens) {
if (isOperator(s)) {
if (stack.size() < 2) {
return 0;
}
int b = stack.pop();
int a = stack.pop();
int result = getResult(a, b, s);
stack.push(result);
} else {
int c = Integer.valueOf(s);
stack.push(c);
}
}
return stack.size() != 1 ? 0 : stack.pop();
}
private boolean isOperator(String s) {
if (s == null || s.length() > 1) {
return false;
}
char c = s.charAt(0);
return c == '+' ||
c == '-' ||
c == '*' ||
c == '/';
}
private int getResult(int a, int b, String c) {
char op = c.charAt(0);
switch (op) {
case '+':
return a + b;
case '-':
return a - b;
case '*':
return a * b;
case '/':
return a / b;
default:
return 0;
}
}
@Test
public void test() {
String[] tokens = new String[]{"2", "1", "+", "3", "*"};
assertSame(9, evalRPN(tokens));
}
}
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