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calculator2.java
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import java.util.Stack;
public class calculator2 {
public static 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;
}
public static void main(String[] args) {
System.out.println(calculate("3+2*2"));
}
}
// time complexity - O(n)
// space complexity - O(n)