/*
*150. Evaluate Reverse Polish Notation
* 1.1 by Mingyang
* 这里我开始想的是把string变成char再来判断是数字还是符号,但是我发现错了!因为这里string变成char非常难!!!
* 所以Integer.parseInt(tokens[i]就可以很好的解决这个问题,直接变为int,包括你的符号!!!!
*/
public int evalRPN(String[] tokens) {
Stack<Integer> stack = new Stack<Integer>();
int a, b;
for (int i = 0; i < tokens.length; i++) {
if (tokens[i].equals("+")) {
a = stack.pop();
b = stack.pop();
stack.push(b + a);
} else if (tokens[i].equals("-")) {
a = stack.pop();
b = stack.pop();
stack.push(b - a);
} else if (tokens[i].equals("*")) {
a = stack.pop();
b = stack.pop();
stack.push(b * a);
} else if (tokens[i].equals("/")) {
a = stack.pop();
b = stack.pop();
stack.push(b / a);
} else {
stack.push(Integer.parseInt(tokens[i]));
}
}
return stack.pop();
}