227. Basic Calculator II

https://leetcode.com/problems/basic-calculator-ii/

Problem

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

Example 1:

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

Example 2:

Input: " 3/2 "
Output: 1

Example 3:

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

Note:

• You may assume that the given expression is always valid.

• Do not use the eval built-in library function.

Solution

class Solution {
public:
    int calculate(string s) {
        if (s.empty()) return 0;
        int i = 0;
        int intermediate = next(s, i);
        int result = 0;
        while (i < s.size()) {
            const char op = s[i];
            const int next_num = next(s, ++i);
            switch (op) {
                case '+':
                    result += intermediate;
                    intermediate = next_num;
                    break;
                case '*':
                    intermediate *= next_num;
                    break;
                case '/':
                    intermediate /= next_num;
                    break;
                default:
                    result += intermediate;
                    intermediate = -next_num;
            }
            
        }
        result += intermediate;
        return result;
    }
    inline int next(string &s, int &idx) {
        int num = 0;
        while (idx < s.size() && isdigit(s[idx]) || isspace(s[idx])) {
            if (isdigit(s[idx])) num = num * 10 + (s[idx] - '0');
            ++idx;
        }
        return num;
    }
};

• #math

• #important