basicCalculatorII.java

// 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.

// Example 1:

// Input: "3+2*2"
// Output: 7
// Example 2:

// Input: " 3/2 "
// Output: 1
// Example 3:

// Input: " 3+5 / 2 "
// Output: 5


USE A STACK 

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 re = 0;
    for(int i:stack){
        re += i;
    }
    return re;
}
}