leetcode150 Evaluate Reverse Polish Notation

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

计算逆波兰表达式，使用一个栈来存储操作数字，遍历输入的链表。

如果为数字，直接push到栈内，如果为操作符号，就把栈顶的两个元素pop出来运算一下。

运算的结果push到栈内。

public class Solution {
public int evalRPN(String[] tokens) {
int result = 0;
Stack<Integer> stack = new Stack<Integer>();
String operators = "+-*/";

for(int i=0; i<tokens.length;i++){
if (operators.contains(tokens[i])){
int b = stack.pop();
int a = stack.pop();
stack.push(calculate(a,b,tokens[i]));
}else{
stack.push(Integer.valueOf(tokens[i]));
}
}
result = stack.pop();
return result;
}

public int calculate(int a, int b, String operator){
char op = operator.charAt(0);
switch (op) {
case '+': return a + b;
case '-': return a - b;
case '*': return a * b;
case '/': return a / b;
}
return 0;
}
}