Question

150. Evaluate Reverse Polish Notation

中文文档

Description

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return _an integer that represents the value of the expression_.

Note that:

• The valid operators are '+', '-', '*', and '/'.
• Each operand may be an integer or another expression.
• The division between two integers always truncates toward zero.
• There will not be any division by zero.
• The input represents a valid arithmetic expression in a reverse polish notation.
• The answer and all the intermediate calculations can be represented in a 32-bit integer.

 

Example 1:

Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
Output: 22
Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

 

Constraints:

• 1 <= tokens.length <= 104
• tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

Solutions

Solution 1

import operator


class Solution:
  def evalRPN(self, tokens: List[str]) -> int:
    opt = {
      "+": operator.add,
      "-": operator.sub,
      "*": operator.mul,
      "/": operator.truediv,
    }
    s = []
    for token in tokens:
      if token in opt:
        s.append(int(opt[token](s.pop(-2), s.pop(-1))))
      else:
        s.append(int(token))
    return s[0]

class Solution {
  public int evalRPN(String[] tokens) {
    Deque<Integer> stk = new ArrayDeque<>();
    for (String t : tokens) {
      if (t.length() > 1 || Character.isDigit(t.charAt(0))) {
        stk.push(Integer.parseInt(t));
      } else {
        int y = stk.pop();
        int x = stk.pop();
        switch (t) {
        case "+":
          stk.push(x + y);
          break;
        case "-":
          stk.push(x - y);
          break;
        case "*":
          stk.push(x * y);
          break;
        default:
          stk.push(x / y);
          break;
        }
      }
    }
    return stk.pop();
  }
}

class Solution {
public:
  int evalRPN(vector<string>& tokens) {
    stack<int> stk;
    for (auto& t : tokens) {
      if (t.size() > 1 || isdigit(t[0])) {
        stk.push(stoi(t));
      } else {
        int y = stk.top();
        stk.pop();
        int x = stk.top();
        stk.pop();
        if (t[0] == '+')
          stk.push(x + y);
        else if (t[0] == '-')
          stk.push(x - y);
        else if (t[0] == '*')
          stk.push(x * y);
        else
          stk.push(x / y);
      }
    }
    return stk.top();
  }
};

func evalRPN(tokens []string) int {
  // https://github.com/emirpasic/gods#arraystack
  stk := arraystack.New()
  for _, token := range tokens {
    if len(token) > 1 || token[0] >= '0' && token[0] <= '9' {
      num, _ := strconv.Atoi(token)
      stk.Push(num)
    } else {
      y := popInt(stk)
      x := popInt(stk)
      switch token {
      case "+":
        stk.Push(x + y)
      case "-":
        stk.Push(x - y)
      case "*":
        stk.Push(x * y)
      default:
        stk.Push(x / y)
      }
    }
  }
  return popInt(stk)
}

func popInt(stack *arraystack.Stack) int {
  v, _ := stack.Pop()
  return v.(int)
}

function evalRPN(tokens: string[]): number {
  const stack = [];
  for (const token of tokens) {
    if (/\d/.test(token)) {
      stack.push(Number(token));
    } else {
      const a = stack.pop();
      const b = stack.pop();
      switch (token) {
        case '+':
          stack.push(b + a);
          break;
        case '-':
          stack.push(b - a);
          break;
        case '*':
          stack.push(b * a);
          break;
        case '/':
          stack.push(~~(b / a));
          break;
      }
    }
  }
  return stack[0];
}

impl Solution {
  pub fn eval_rpn(tokens: Vec<String>) -> i32 {
    let mut stack = vec![];
    for token in tokens {
      match token.parse() {
        Ok(num) => stack.push(num),
        Err(_) => {
          let a = stack.pop().unwrap();
          let b = stack.pop().unwrap();
          stack.push(match token.as_str() {
            "+" => b + a,
            "-" => b - a,
            "*" => b * a,
            "/" => b / a,
            _ => 0,
          });
        }
      }
    }
    stack[0]
  }
}

using System.Collections.Generic;

public class Solution {
  public int EvalRPN(string[] tokens) {
    var stack = new Stack<int>();
    foreach (var token in tokens)
    {
      switch (token)
      {
        case "+":
          stack.Push(stack.Pop() + stack.Pop());
          break;
        case "-":
          stack.Push(-stack.Pop() + stack.Pop());
          break;
        case "*":
          stack.Push(stack.Pop() * stack.Pop());
          break;
        case "/":
          var right = stack.Pop();
          stack.Push(stack.Pop() / right);
          break;
        default:
          stack.Push(int.Parse(token));
          break;
      }
    }
    return stack.Pop();
  }
}

Solution 2

class Solution:
  def evalRPN(self, tokens: List[str]) -> int:
    nums = []
    for t in tokens:
      if len(t) > 1 or t.isdigit():
        nums.append(int(t))
      else:
        if t == "+":
          nums[-2] += nums[-1]
        elif t == "-":
          nums[-2] -= nums[-1]
        elif t == "*":
          nums[-2] *= nums[-1]
        else:
          nums[-2] = int(nums[-2] / nums[-1])
        nums.pop()
    return nums[0]