Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.
Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().
Example 1:
Input: s = "1 + 1" Output: 2
Example 2:
Input: s = " 2-1 + 2 " Output: 3
Example 3:
Input: s = "(1+(4+5+2)-3)+(6+8)" Output: 23
Constraints:
1 <= s.length <= 3 * 105s consists of digits, '+', '-', '(', ')', and ' '.s represents a valid expression.'+' is not used as a unary operation (i.e., "+1" and "+(2 + 3)" is invalid).'-' could be used as a unary operation (i.e., "-1" and "-(2 + 3)" is valid).class Solution {
public:
int calculate(string s) {
stack<int> op({1});
int total = 0, num = 0;
int isPositive = 1;
for (char c : s) {
if (isdigit(c)) {
num = num * 10 + (c - '0');
} else {
total += num * isPositive * op.top();
num = 0;
if (c == '+') isPositive = 1;
if (c == '-') isPositive = -1;
if (c == ')') op.pop();
if (c == '(') {
op.push(op.top() * isPositive);
isPositive = 1;
}
}
}
total += num * isPositive * op.top();
return total;
}
};