Question Definition
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.
Java 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 re = 0;
for(int i:stack){
re += i;
}
return re;
}
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