Implement a basic calculator to evaluate a simple expression string.
The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.
You may assume that the given expression is always valid.
Some examples:
"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5
Note: Do not use the eval built-in library function.
代码如下：
public class Solution {
public int calculate(String s) {
int len;
if(s==null || (len=s.length())==0) return 0;
Stack<Integer> stack=new Stack<Integer>();
int num=0;
char sign='+';
for(int i=0;i<len;i++){
if(Character.isDigit(s.charAt(i))){
num=num*10+s.charAt(i)-'0';
}
if(!Character.isDigit(s.charAt(i)) && ' '!=s.charAt(i) || i==len-1){
if(sign=='-'){
stack.push(-num);
}
if(sign=='+'){
stack.push(num);
}
if(sign=='/'){
stack.push(stack.pop()/num);
}
if(sign=='*'){
stack.push(stack.pop()*num);
}
sign=s.charAt(i);
num=0;
}
}
int ret=0;
for(int i:stack){
ret+=i;
}
return ret;
}
}
运行结果：