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Basic Calculator

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Description

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

Solution

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class Solution {
public:
    int calculate(string s) {
        int index = 0;
        return this->calculateFrom(index, s);
    }
    
    int calculateFrom(int& index, string& s) {
        int ret = 0;
        int sign = 1;
        while(index < s.size()) {
            while(s[index] == ' ') index += 1;
            switch (s[index]) {
                case '-':
                    index += 1;
                    sign = -1;
                    break;
                case '(': 
                    index += 1;
                    ret += sign * this->calculateFrom(index, s);
                    continue;
                case ')':
                    index += 1;
                    return ret;
                case '+':
                    index += 1;
                    sign = 1;
                    continue;
                default:
                //Number
                ret += sign * this->extractNumber(s, index);
            }
        }
        return ret;
    }
    
    int extractNumber(string& s, int& index) {
        int ret = 0;
        while (index < s.size() && s[index] <= '9' && s[index] >= '0') {
            ret = ret * 10 + (s[index] - '0');
            index += 1;
        }
        return ret;
    }
};