leetcode.150 evaluate reverse polish notation

题意

• 计算逆波兰法表示的算式

  解法

• 遇到数字存入栈，遇到符号则出栈两个数字，并将结果入栈

  class Solution {
  Stack<Integer> integers = new Stack<>();
  public int evalRPN(String[] tokens) {
  
  for(String token: tokens) {
  
  if ("+".equals(token)) {
  
  int n2 = integers.pop(), n1 = integers.pop();
  
  integers.push(n1 + n2);
  
  } else if ("-".equals(token)) {
  
  int n2 = integers.pop(), n1 = integers.pop();
  
  integers.push(n1 - n2);
  
  } else if ("*".equals(token)) {
  
  int n2 = integers.pop(), n1 = integers.pop();
  
  integers.push(n1 * n2);
  
  } else if ("/".equals(token)) {
  
  int n2 = integers.pop(), n1 = integers.pop();
  
  integers.push(n1 / n2);
  
  } else {
  
  integers.push(Integer.parseInt(token));
  
  }
  
  }
  
  return integers.pop();
  
  }
  
  }