Q:计算逆波兰式(后缀表达式)的值
运算符仅包含"+","-","*"和"/",被操作数可能是整数或其他表达式
例如:
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9↵ ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
A:使用栈,碰到符号就跳出来两个值计算再压入结果
import java.util.Stack;
public int evalRPN(String[] tokens) {
Stack<Integer> stack = new Stack();
for (int i = 0; i < tokens.length; i++) {
try {
int num = Integer.parseInt(tokens[i]);
stack.push(num);
} catch (Exception e) {
int a = stack.pop();
int b = stack.pop();
if (tokens[i].equals("+"))
stack.push(a + b);
else if (tokens[i].equals("-"))
stack.push(b - a);
else if (tokens[i].equals("*"))
stack.push(a * b);
else if (tokens[i].equals("/"))
stack.push(b / a);
}
}
return stack.pop();
}