（leetcode)Evaluate Reverse Polish Notation

Evaluate Reverse Polish Notation

题目描述：

Evaluate the value of an arithmetic expression in Reverse Polish Notation.
　　Valid operators are +, -, *, /. Each operand may be an integer or another expression.
　　Some examples: ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9 ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
题目分析：给定一个逆波兰表达式，求该表达式的值。
public int evalRPN(String[] tokens){}
传递参数为字符串数组，只需申请一个栈，并遍历数组，若数组是数字，就将数字压入栈，若是运算符，则弹出栈中的两个数字进行相应运算，并将计算结果压入栈中，直到遍历结束。最后返回栈顶元素即可。

public class Solution {
public int evalRPN(String[] tokens) {
Stack<Integer> stack=new Stack<Integer>();
for(int i=0;i<tokens.length;i++){
if(tokens[i].equals("+")){
int second=stack.pop();
int first=stack.pop();
int re=first+second;
stack.push(re);
}
else if(tokens[i].equals("-")){
int second=stack.pop();
int first=stack.pop();
int re=first-second;
stack.push(re);
}
else if(tokens[i].equals("*")){
int second=stack.pop();
int first=stack.pop();
int re=first*second;
stack.push(re);
}
else if(tokens[i].equals("/")){
int second=stack.pop();
int first=stack.pop();
int re=first/second;
stack.push(re);
}
else{
stack.push(Integer.parseInt(tokens[i]));
}
}
return stack.pop();
}
}