224. Basic Calculator
1. Description
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().
2. Example
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
3. Constraints
- 1 <= s.length <= 3 * 10$^5$
- s 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).
- There will be no two consecutive operators in the input.
- Every number and running calculation will fit in a signed 32-bit integer.
4. Solutions
Stack
n = expression.size()
Time complexity: O(n)
Space complexity: O(n)
class Solution {
public:
int calculate(const string &expression) {
int64_t result = 0, number = 0, sign = 1;
stack<int> results, signs;
for (char c : expression) {
if (isdigit(c)) {
number = number * 10 + c - '0';
} else if (c == '+') {
result += sign * number;
sign = 1;
number = 0;
} else if (c == '-') {
result += sign * number;
sign = -1;
number = 0;
} else if (c == '(') {
results.push(result);
signs.push(sign);
result = 0;
sign = 1;
number = 0;
} else if (c == ')') {
result += sign * number;
number = 0;
result *= signs.top();
signs.pop();
result += results.top();
results.pop();
}
}
return result + sign * number;
}
};