Basic Calculator II

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.

Solution:

public class Solution {
    public int calculate(String s) {
        char sign = '+';
        int result = 0;
        int preVal = 0;
        for(int i=0; i<s.length(); i++){
            char c = s.charAt(i);
            if(c>='0' && c<='9'){
                int num = s.charAt(i) - '0';
                while(i+1<s.length() && s.charAt(i+1)>='0' && s.charAt(i+1)<='9'){
                    num = num*10 + (s.charAt(i+1)-'0');
                    i++;
                }
                if(sign=='+'){
                    result += num;
                    preVal = num;
                } else if(sign=='-'){
                    result -= num;
                    preVal = -num;
                } else if(sign=='*'){
                    result = result - preVal + preVal * num;
                    preVal = preVal * num;
                } else if(sign=='/'){
                    result = result - preVal + preVal / num;
                    preVal = preVal / num;
                }
            } else if(c=='+'){
                sign = '+';
            } else if(c=='-'){
                sign = '-';
            } else if(c=='*'){
                sign = '*';
            } else if(c=='/'){
                sign = '/';
            }
        }
        return result;
    }
}