LeetCode : 227. Basic Calculator II

public int calculate(String s) {
    long ans = 0L;

    Stack<Long> valueStack = new Stack<>();

    s = s.replaceAll(" ", "").toLowerCase(); // [1]
    char[] chars = s.toCharArray();

    char operation = '+';
    Long curValue = 0L;
    for (int i = 0; i < chars.length; i++) {

        if (48 <= chars[i] && chars[i] <= 57) { // [2-1]
            curValue = (curValue * 10) + (chars[i] - '0'); // [3]
        }

        if (i == chars.length - 1
            || !('0' <= chars[i] && chars[i] <= '9')) { // [2-2]

            if (operation == '+') {
                valueStack.push(curValue);
            } else if (operation == '-') {
                valueStack.push(-curValue);
            } else if (operation == '*') {
                valueStack.push(valueStack.pop() * curValue);
            } else if (operation == '/') {
                valueStack.push(valueStack.pop() / curValue);
            }

            operation = chars[i];
            curValue = 0L;
        }
    }

    while (!valueStack.isEmpty()) {
        ans += valueStack.pop();
    }
    return (int) ans;
}