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 .
You may assume that the given expression is always valid.
Some examples:
"1 + 1" = 2 " 2-1 + 2 " = 3 "(1+(4+5+2)-3)+(6+8)" = 23
Note: Do not use the eval built-in library function.
代码如下：
public class Solution {
public int calculate(String s) {
Stack<Integer>stack=new Stack<Integer>();
int ret=0;
int num=0;
int sign=1;
for(int i=0;i<s.length();i++){
char c=s.charAt(i);
if(Character.isDigit(c)){
num=10*num+c-'0';
}
else if(c=='+'){
ret+=sign*num;
num=0;
sign=1;
}else if(c=='-'){
ret+=sign*num;
num=0;
sign=-1;
}else if(c=='('){
stack.push(ret);
stack.push(sign);
sign=1;
ret=0;
}else if(c==')'){
ret+=sign*num;
num=0;
ret*=stack.pop();
ret+=stack.pop();
}
}
if(num!=0) ret+=sign*num;
return ret;
}
}
运行结果：
