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 * 105
- 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.
class Solution {
public int calculate(String s) {
if(s==null){
return 0;
}
int result = 0;
int sign = 1;
int num = 0;
Stack<Integer> stack = new Stack<Integer>();
stack.push(sign);
for(int i = 0; i < s.length(); i++) {
char c = s.charAt(i);
if(c >= '0' && c <= '9') {
num = num * 10 + (c - '0');
}else if(c == '+' || c == '-') {
result += sign * num;
sign = stack.peek() * (c == '+' ? 1: -1);
num = 0;
}else if(c == '(') {
stack.push(sign);
}else if(c == ')') {
stack.pop();
}
}
result += sign * num;
return result;
}
}