Leetcode Basic Calculator I && II

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2" 2-1 + 2 " = 3"(1+(4+5+2)-3)+(6+8)" = 23

    public int calculate(String s) {        if (s==null || s.length() == 0) return 0;          Stack<Integer> st = new Stack<Integer>();          int res = 0, sign = 1, len = s.length();        for (int i = 0; i < len; i++) {            char c = s.charAt(i);            if (Character.isDigit(c)) {                int cur = c - '0';                while (i + 1 < len && Character.isDigit(s.charAt(i + 1))) {                    cur = 10 * cur + s.charAt(++i) - '0';                }                res += sign * cur;            }            else if (c == '-') sign = -1;            else if (c == '+') sign = 1;            else if (c == '(') {                st.push(res);                res = 0;                st.push(sign);                sign = 1;            }            else if (c == ')') {                res = st.pop() * res + st.pop();                sign = 1;            }                    }        return res;

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

public int calculate(String s) {        Stack<Integer> stack = new Stack<>();        char sign = '+';        int len = s.length();        int n = 0;        for (int i = 0; i < len; i++) {            char c = s.charAt(i);            if (Character.isDigit(c)) {                n = n * 10 + c - '0';            }            if (!Character.isDigit(c) && c != ' ' || i == len - 1) {                if (sign == '+') {                    stack.push(n);                } else if (sign == '-') {                    stack.push(-n);                } else if (sign == '*') {                    stack.push(stack.pop() * n);                } else if (sign == '/') {                    stack.push(stack.pop() / n);                }                sign = c;                n = 0;            }        }        int re = 0;        while (!stack.isEmpty()) re += stack.pop();        return re;    }