leetcode: 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.

class Solution {
public:
int calculate(string s) {

stack<int> signs;

int lastSign = 1;
int curNum = 0;
int retSum = 0;

for (auto c: s)
{
if (c == '(')
{
signs.push(lastSign);//进栈主要用在有括号的情况，如果没有括号，就不需要有栈
}
else if (c == ')')
{
signs.pop();
}
else if (c >= '0' && c <= '9')
{
curNum = curNum * 10 + (c-'0');
}
else if (c == '+' || c == '-')
{
retSum = retSum + lastSign * curNum;
lastSign = (c == '+'? 1:-1);
if (signs.size() > 0)
lastSign = lastSign * signs.top();
curNum = 0;
}
}

return retSum + lastSign * curNum;
}
};