Description
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 .
Note:
You may assume that the given expression is always valid.
Do not use the eval built-in library function.
Constraints
Approach
Links
Examples
Input: " 2-1 + 2 "
Output: 3
Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23
Solutions
/**
* Time complexity :
* Space complexity :
*/
class Solution {
public int calculate(String s) {
Stack<Integer> stack = new Stack<>();
int n = s.length(), sign = 1, result = 0;
for(int i = 0; i < n; i++) {
if(Character.isDigit(s.charAt(i))) {
int sum = s.charAt(i) - '0';
while(i+1 < n && Character.isDigit(s.charAt(i+1))) {
sum = sum * 10 + (s.charAt(i+1) - '0');
i++;
}
result += sum * sign;
} else if(s.charAt(i) == '+') {
sign = 1;
} else if(s.charAt(i) == '-') {
sign = -1;
} else if(s.charAt(i) == '(') {
stack.push(result);
stack.push(sign);
result = 0;
sign = 1;
} else if(s.charAt(i) == ')') {
result = result * stack.pop() + stack.pop();
}
}
return result;
}
}
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