Basic Calculator - LintCode

描述
Implement a basic calculator to evaluate a simple expression string.
The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .
You may assume that the given expression is always valid.

Do not use the eval built-in library function.

樣例

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

思路

#ifndef C978_H
#define C978_H
#include<iostream>
#include<string>
#include<vector>
#include<stack>
using namespace std;
class Solution {
public:
    /**
    * @param s: the given expression
    * @return: the result of expression
    */
    int calculate(string &s) {
        // Write your code here
        if (s.empty())
            return 0;
        vector<string> expression = getExpression(s);
        vector<string> post = postExpression(expression);
        stack<string> stack;
        for (auto c : post)
        {
            if (c == "+" || c == "-")
            {
                int x = stoi(stack.top());
                stack.pop();
                int y = stoi(stack.top());
                stack.pop();
                int num = 0;
                if (c == "+")
                    num = y + x;
                else
                    num = y - x;
                stack.push(to_string(num));
            }
            else
                stack.push(c);
        }
        if (stack.empty())
            return 0;
        else
            return stoi(stack.top());
    }
    //提取字符串中的內容,構建表達式
    vector<string> getExpression(string &s)
    {
        vector<string> expression;
        string str;
        for (auto c : s)
        {
            if (c == '+' || c == '-' || c == '('|| c== ')')
            {
                if (!str.empty())
                {
                    expression.push_back(str);
                    str.clear();
                }
                str += c;
                expression.push_back(str);
                str.clear();
            }
            else if (isdigit(c))
            {
                str += c;
            }
        }
        if (!str.empty())
            expression.push_back(str);
        return expression;
    }
    //獲得字符串的優先級
    int getPriority(string str)
    {
        if (str == "(")
            return 0;
        else if (str == "+" || str == "-")
            return 1;
        else
            return 2;
    }
    //將表達式轉化爲逆波蘭表達式
    vector<string> postExpression(vector<string> &expression)
    {
        vector<string> post;
        stack<string> stack;
        for (auto c : expression)
        {
            if (c == "(")
                stack.push(c);
            else if (c == ")")
            {
                while (stack.top() != "(")
                {
                    post.push_back(stack.top());
                    stack.pop();
                }
                stack.pop();
            }
            else
            {
                while (!stack.empty() && getPriority(stack.top()) >= getPriority(c))
                {
                    post.push_back(stack.top());
                    stack.pop();
                }
                stack.push(c);
            }
        }
        while (!stack.empty())
        {
            post.push_back(stack.top());
            stack.pop();
        }
        return post;
    }
};
#endif