leetcode 150. Evaluate Reverse Polish Notation 逆波兰表达式的计算

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

本题题意就是根据给定的表达式计算结果，就是后序计算表达式。

代码如下：

import java.util.Stack;/* * 逆波兰表达式计算，就是后序 * */public class Solution {    public int evalRPN(String[] tokens)     {        if(tokens==null || tokens.length<=0)            return 0;        Stack<Integer> myStack=new Stack<>();        for(int i=0;i<tokens.length;i++)        {            String one=tokens[i];            if(isOperator(one))            {                //记着要a和b的操作数的顺序，对于 整数除法是相关的，这里也不压考虑分母为0的情况                int a=myStack.pop();                int b=myStack.pop();                int res=getRes(b,a,one);                myStack.push(res);            }else                myStack.push(Integer.parseInt(one));        }        int finRes=myStack.pop();        return finRes;    }    public int getRes(int a, int b, String one)     {        one=one.replace(" ", "");        if(one.equals("+"))            return (a+b);        else if(one.equals("-"))            return (a-b);        else if(one.equals("*"))            return (a*b);        else if(one.equals("/"))            return (a/b);        return 0;    }    public boolean isOperator(String one)     {        one=one.replace(" ", "");        if(one.equals("+") || one.equals("-") || one.equals("*") || one.equals("/") )            return true;        else            return false;    }}

下面是C++的做法，就是使用栈做遍历

代码如下：

#include <iostream>#include <climits>#include <map>#include <vector>#include <algorithm>#include <stack>#include <sstream>using namespace std;class Solution {public:    int evalRPN(vector<string>& tokens)     {        if (tokens.size() <= 0)            return 0;        stack<int> skt;        for (int i = 0; i < tokens.size(); i++)        {            if (isOp(tokens[i]))            {                int a = skt.top();                skt.pop();                int b = skt.top();                skt.pop();                skt.push(getRes(b, a, tokens[i]));            }            else            {                stringstream ss;                ss << tokens[i];                int a = 0;                ss >> a;                skt.push(a);            }        }        int res = skt.top();        skt.pop();        return res;    }    int getRes(int a, int b, string op)    {        if (op == "+")            return a + b;        else if (op == "-")            return a - b;        else if (op == "*")            return a * b;        else if (op == "/")            return a / b;        else            return 0;    }    bool isOp(string op)    {        if (op == "+" || op == "-" || op == "*" || op == "/")            return true;        else            return false;    }};