Title Description

Implement a basic calculator to evaluate the value of a simple string expression.

The string expression contains only four operators and spaces: nonnegative integer, +, -, *, / and. Integer division keeps only the integer part.

Example 1:

Input: "3 + 2 * 2"
Output: 7
Example 2:

Input: "3 / 2"
Output: 1
Example 3:

Input: "3 + 5 / 2"
Output: 5
Explain:

You can assume that all the expressions given are valid.
Do not use the built-in library function eval.

Source: LeetCode
Link: https://leetcode-cn.com/problems/basic-calculator-ii
Copyright belongs to the network. For commercial reprint, please contact the official authorization. For non-commercial reprint, please indicate the source.

Solving problems

First convert infix expression to suffix expression, and then evaluate suffix expression. Code runs extremely slowly...

class Solution {
public:
    int calculate(string s) {
        string ss = "";
        for(int i=0;i<s.length();i++){
            if(s[i] != ' ') ss+=s[i];
        }
        vector<string> vect;
        stack<int> ans;
        stack<string> sk;
        string decline = "+-/*";
        map<string,int> mp = {{"+",0},{"-",0},{"*",1},{"/",1}};
        string::size_type st1,st2;
        st1 = ss.find_first_not_of(decline);
        while(st1 != string::npos){
            st2 = ss.find_first_of(decline,st1);
            if(st2 == string::npos) st2 = ss.length();
            string num = ss.substr(st1,st2-st1);
            vect.push_back(num);
            st1 = ss.find_first_not_of(decline,st2);
            if(st1 != string::npos){
                string op = ss.substr(st2,st1-st2);
                while(!sk.empty() && mp[sk.top()] >= mp[op]){
                    vect.push_back(sk.top());
                    sk.pop();
                }
                sk.push(op);
            }
        }
        while(!sk.empty()){
            vect.push_back(sk.top());
            sk.pop();
        }
        int num1,num2;
        for(auto it : vect){
            if(it == "+"){
                num2 = ans.top();
                ans.pop();
                num1 = ans.top();
                ans.pop();
                ans.push(num1+num2);
            }
            else if(it == "-"){
                num2 = ans.top();
                ans.pop();
                num1 = ans.top();
                ans.pop();
                ans.push(num1-num2);
            }
            else if(it == "*"){
                num2 = ans.top();
                ans.pop();
                num1 = ans.top();
                ans.pop();
                ans.push(num1*num2);
            }
            else if(it == "/"){
                num2 = ans.top();
                ans.pop();
                num1 = ans.top();
                ans.pop();
                ans.push(num1/num2);
            }
            else ans.push(atoi(it.c_str()));
        }
        return ans.top();
    }
};