public class CalculatorI {

public int calculate(String s) {

if(s == null || s.length() == 0) return 0;

char[] ch = s.toCharArray();

int le = ch.length, i = 0;

// skip all preceding empty space

while(i < le && ch[i] == ' ') ++i;

// if it reaches the end of string, return 0, meaning there is nothing inside the expression.

if(i == le) return 0;

int first = 0;

while(i < le && Character.isDigit(ch[i])) first = first * 10 + (ch[i++] - '0');

// if expression is filled with digits, just return it as the result.

if(i == le) return first;

// else let it to the first operand.

// prev is used to track the last operand visited so far.

int rs = first, prev = first;

while(i < le) {

while(i < le && ch[i] == ' ') ++i;

if(i == le) break;

int n2 = 0;

if(ch[i] == '+') {

while(i < le && !Character.isDigit(ch[i])) ++i;

while(i < le && Character.isDigit(ch[i])) n2 = n2 * 10 + (ch[i++] - '0');

rs += n2;

prev = n2;

} else if(ch[i] == '-') {

while(i < le && !Character.isDigit(ch[i])) ++i;

while(i < le && Character.isDigit(ch[i])) n2 = n2 * 10 + (ch[i++] - '0');

rs -= n2;

prev = -n2;

} else if(ch[i] == '*') {

while(i < le && !Character.isDigit(ch[i])) ++i;

while(i < le && Character.isDigit(ch[i])) n2 = n2 * 10 + (ch[i++] - '0');

rs = rs - prev + prev * n2;

prev = prev * n2;

} else if(ch[i] == '/') {

while(i < le && !Character.isDigit(ch[i])) ++i;

while(i < le && Character.isDigit(ch[i])) n2 = n2 * 10 + (ch[i++] - '0');

rs = rs - prev + prev / n2;

prev = prev / n2;

}

//System.out.println(rs);

}

return rs;

}

}