leetcode(150):Evaluate Reverse Polish Notation

原题：

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

分析：题意为计算逆波兰表达式的值，用stack很好解决，直接上代码：

public class Solution {
public int evalRPN(String[] tokens) {
Stack<Integer> st = new Stack<Integer>();
for (String s:tokens) {
if(s.equals("+"))
{
st.push(st.pop()+st.pop());
}
else if(s.equals("-"))
{
int a =st.pop();
int b = st.pop();
st.push(b-a);
}
else if(s.equals("*"))
{
st.push(st.pop()*st.pop());
}
else if(s.equals("/"))
{
int a = st.pop();
int b = st.pop();
st.push(b/a);
}
else{
st.push(Integer.parseInt(s));
}
}
return st.pop();
}
}