leetcode 224. Basic Calculator 、227. Basic Calculator II

这种题都要设置一个符号位的变量

224. Basic Calculator

设置数值和符号两个变量，遇到左括号将数值和符号加进栈中

class Solution {
public:
    int calculate(string s) {
        stack<int> st;
        int res = ,flag = ;
        for(int i = ;i < s.size();i++){
            if(s[i] >= '' && s[i] <= ''){
                int num = ;
                while(i < s.size() && s[i] >= '' && s[i] <= ''){
                    num = num* + flag * (s[i] - '');
                    i++;
                }
                res += num;
                i--;
            }
            else if(s[i] == '+')
                flag = ;
            else if(s[i] == '-')
                flag = -;
            else if(s[i] == '('){
                st.push(res);
                st.push(flag);
                res = ,flag = ;
            }
            else if(s[i] == ')'){
                res *= st.top();
                st.pop();
                res += st.top();
                st.pop();
            }
        }
        return res;
    }
};

227. Basic Calculator II

乘除法有优先级，这个时候需要将这些数值弹出

与上一题不同，这个题是把所有的结果存入进stack中

class Solution {
public:
    int calculate(string s) {
        long res = ,num = ;
        char op = '+';
        stack<int> st;
        for(int i = ;i < s.size();i++){
            if(s[i] >= '' && s[i] <= '')
                num = num *  + s[i] - '';
            if(s[i] == '+' || s[i] == '-' || s[i] == '*' || s[i] == '/' || i == s.size() - ){
                if(op == '+')
                    st.push(num);
                if(op == '-')
                    st.push(-num);
                if(op == '*' || op == '/'){
                    int tmp = op == '*' ? st.top() * num : st.top() / num;
                    st.pop();
                    st.push(tmp);
                }
                op = s[i];
                num = ;
            }
        }
        while(!st.empty()){
            res += st.top();
            st.pop();
        }
        return res;
    }
};