Leetcode-224: basic calculator

subject

Topic linking
Here's a string expression for you s , Please implement a basic calculator to calculate and return its value .

Be careful : Any built-in functions that evaluate strings as mathematical expressions are not allowed , such as eval() .

Example 1：

 Input ：s = "1 + 1"
 Output ：2

Example 2：

 Input ：s = " 2-1 + 2 "
 Output ：3

Example 3：

 Input ：s = "(1+(4+5+2)-3)+(6+8)"
 Output ：23

Problem solving

Method 1 ： Stack

Reference link

class Solution {
    
public:
    int calculate(string s) {
    
        int res=0,num=0,sign=1;
        stack<int> st;
        for(char c:s){
    
            if(c>='0'&&c<='9'){
    
                num=num*10+(c-'0');
            }
            else if(c=='+'||c=='-'){
    
                res+=sign*num;
                num=0;
                sign=c=='+'?1:-1;
            }
            else if(c=='('){
    
                st.push(res);
                st.push(sign);
                res=0;
                sign=1;
            }
            else if(c==')'){
    
                res+=sign*num;
                num=0;
                res*=st.top();
                st.pop();
                res+=st.top();
                st.pop();
            }
        }
        res+=sign*num;
        return res;

    }
};