224. Basic Calculator

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.

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Stack 存起来就行

public class Solution {
private int calc(Stack<String> stack2){
int ret = 0;
int state = 1;//1为加0为减
while(!stack2.empty()){

4000
int num = 0;
String tem = stack2.pop();
if(tem.equals(")"))break;
else if(tem.equals("("))num=calc(stack2);
else if(tem.equals("+")){state=1;
continue;
}
else if(tem.equals("-")){state=0;
continue;
}else {
num=Integer.parseInt(tem);
}
if(state==1){
ret=ret+num;
}else{
ret=ret-num;
}
}
return ret;
}
public int calculate(String s) {
Stack<String> stack=new Stack<>();
Stack<String> stack2 = new Stack<>();
int length = s.length();
for(int i=length-1;i>=0;){
char k = s.charAt(i);
if(k==' '){
i--;
continue;

}
if(k==')'||k=='('||k=='+'||k=='-'){
String t = ""+s.charAt(i);
i--;
stack2.push(t);
continue;

}
String t = "";
while(i>=0&&(s.charAt(i)==' '||('0'<=s.charAt(i)&&'9'>=s.charAt(i)))){
k=s.charAt(i);
if(k==' '){
i--;
continue;
}
t = k+t;
i--;
}
stack2.push(t);
}
return calc(stack2);
}
}