逆波兰表达式求值
有效的算符包括 +
、-
、*
、/
。每个运算对象可以是整数,也可以是另一个逆波兰表达式。
[https://leetcode-cn.com/problems/evaluate-reverse-polish-
notation/](https://links.jianshu.com/go?to=https%3A%2F%2Fleetcode-
cn.com%2Fproblems%2Fevaluate-reverse-polish-notation%2F)
示例1:
输入:tokens = "2","1","+","3","*"undefined 输出:9undefined 解释:该算式转化为常见的中缀算术表达式为:((2 + 1) * 3) = 9
示例2:
输入:tokens = "4","13","5","/","+"undefined 输出:6undefined 解释:该算式转化为常见的中缀算术表达式为:(4 + (13 / 5)) = 6
示例3:
输入:tokens = "10","6","9","3","+","-11"," ","/"," ","17","+","5","+"undefined 输出:22undefined 解释:undefined 该算式转化为常见的中缀算术表达式为:undefined ((10 (6 / ((9 + 3) -11))) + 17) + 5undefined = ((10 (6 / (12 -11))) + 17) + 5undefined = ((10 (6 / -132)) + 17) + 5undefined = ((10 0) + 17) + 5undefined = (0 + 17) + 5undefined = 17 + 5undefined = 22
提示:
1 <= tokens.length <= 104undefined tokensi 要么是一个算符("+"、"-"、"*" 或 "/"),要么是一个在范围 -200, 200 内的整数
Java解法
思路:
逆波兰表示法就是为了让计算机方便计算使用的,本身就是通过用栈来存储操作数,遇到运算符进行弹出操作数运算再入栈,直到结束 注意踩坑,出栈时的操作数,先出的是被操作的,注意位置顺序
package sj.shimmer.algorithm.m4_2021;
import java.util.Stack;
/**
* Created by SJ on 2021/4/12.
*/
class D75 {
public static void main(String[] args) {
// System.out.println(evalRPN(new String[]{"2","1","+","3","*"}));
System.out.println(evalRPN(new String[]{"4","13","5","/","+"}));
}
public static int evalRPN(String[] tokens) {
Stack<Integer> stack = new Stack<>();
for (String token : tokens) {
Integer a = 0;
Integer b = 0;
switch (token) {
case "+":
b = stack.pop();
a = stack.pop();
stack.add(a + b);
break;
case "-":
b = stack.pop();
a = stack.pop();
stack.add(a - b);
break;
case "*":
b = stack.pop();
a = stack.pop();
stack.add(a * b);
break;
case "/":
b = stack.pop();
a = stack.pop();
stack.add(a / b);
break;
default:
stack.add(Integer.parseInt(token));
break;
}
}
return stack.pop();
}
}
image
官方解
[https://leetcode-cn.com/problems/evaluate-reverse-polish-
notation/solution/ni-bo-lan-biao-da-shi-qiu-zhi-by-leetcod-
wue9/](https://links.jianshu.com/go?to=https%3A%2F%2Fleetcode-
cn.com%2Fproblems%2Fevaluate-reverse-polish-notation%2Fsolution%2Fni-bo-lan-
biao-da-shi-qiu-zhi-by-leetcod-wue9%2F)
- 栈
即我的解法
* 时间复杂度:O(n)
* 空间复杂度:O(n)数组模拟栈
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