[LeetCode]Basic Calculator

Basic Calculator

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

Note: Do not use the eval built-in library function.

用到stack，如果遇到左括号'('将之前的值和1.-1表示运算符号压入stack中，遇到右括号')'，将运算符号和值去除做运算。

class Solution {
public:
    int calculate(string s) {
        if(s.length()==0) return 0;
        stack<int> mystack;
        int result=0;
        int op=1;
        for(int i=0;i<s.length();i++)
        {
            if(s[i]>='0' && s[i]<='9')
            {
                int number=s[i]-'0';
                while((i+1)<s.length() && s[i+1]>='0' && s[i+1]<='9')
                {
                    i++;
                    number=number*10+s[i]-'0';
                }
                result+=op*number;
            }
            if(s[i]==' ')
            {
                continue;
            }
            if(s[i]=='+')
            {
                op=1;
            }
            if(s[i]=='-')
            {
                op=-1;
            }
            if(s[i]=='(')
            {
                mystack.push(result);
                result=0;
                mystack.push(op);
                op=1;
            }
            if(s[i]==')')
            {
                op=mystack.top();
                mystack.pop();
                result=op*result+mystack.top();
                mystack.pop();
                op=1;
            }
        }
        return result;
    }
};