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EvalRPN.java

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package Algorithms.stack;

import java.util.Stack;

public class EvalRPN {

public int evalRPN(String[] tokens) {

if (tokens == null) {

return 0;

}

int len = tokens.length;

Stack<Integer> s = new Stack<Integer>();

for (int i = 0; i < len; i++) {

String str = tokens[i];

if (str.equals("+") || str.equals("-") || str.equals("*") || str.equals("/")) {

// get out the two operation number.

int n2 = s.pop();

int n1 = s.pop();

if (str.equals("+")) {

s.push(n1 + n2);

} else if (str.equals("-")) {

s.push(n1 - n2);

} else if (str.equals("*")) {

s.push(n1 * n2);

} else if (str.equals("/")) {

s.push(n1 / n2);

}

} else {

s.push(Integer.parseInt(str));

}

}

if (s.isEmpty()) {

return 0;

}

return s.pop();

}

}