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 .
Example
No.1
Input: “1 + 1”
Output: 2
No.2
Input: “ 2-1 + 2 “
Output: 3
No.3
Input: “(1+(4+5+2)-3)+(6+8)”
Output: 23
Note
- You may assume that the given expression is always valid.
- Do not use the eval built-in library function.
Code
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public int (String s) {
Stack<Integer> stack = new Stack<>();
int result = 0;
int num = 0;
int sign = 1;
for (int i = 0; i < s.length(); i++) {
char ch = s.charAt(i);
if (Character.isDigit(ch)) {
num = 10 * num + (ch - '0');
}
else if (ch == '+' || ch == '-') {
result += sign * num;
num = 0;
sign = ch == '+' ? 1 : -1;
}
else if (ch == '(') {
stack.push(result);
stack.push(sign);
sign = 1;
result = 0;
}
else if (ch == ')') {
result += sign * num;
num = 0;
result *= stack.pop();
result += stack.pop();
}
}
result += sign * num;
return result;
}
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