# 227. Basic Calculator II

Basic Calculator II - LeetCode

Given a string s which represents an expression, evaluate this expression and return its value. The integer division should truncate toward zero. You may assume that the given expression is always valid. All intermediate results will be in the range of [-231, 231 - 1]. Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Example 1: Input: s = “3+2*2” Output: 7 Example 2: Input: s = " 3/2 " Output: 1 Example 3: Input: s = " 3+5 / 2 " Output: 5

Constraints:

1 <= s.length <= 3 * 105 s consists of integers and operators ('+', ‘-’, ‘*’, ‘/') separated by some number of spaces. s represents a valid expression. All the integers in the expression are non-negative integers in the range [0, 231 - 1]. The answer is guaranteed to fit in a 32-bit integer.

• code
``````class Solution {
public int calculate(String s) {
char lastOp = '+';
int lastNum = 0;
int res = 0;
int curNum = 0;

for (int i = 0; i < s.length(); i++){
char cur = s.charAt(i);
if (Character.isDigit(cur)){
curNum = curNum * 10 + (cur - '0');
}
if (!Character.isDigit(cur) && !Character.isWhitespace(cur) || i == s.length() - 1){
if (lastOp == '+' || lastOp == '-'){
res += lastNum;
lastNum = (lastOp == '+') ? curNum : -curNum; // check lastOp here
}
else{
if (lastOp == '*'){
lastNum *= curNum;
}else{
lastNum /= curNum;
}
}
lastOp = cur;
curNum = 0; // don't forget
}
}
res += lastNum; // don't forget
return res;
}
}
``````
• code naive deque…
``````class Solution:
def calculate(self, s: str) -> int:
num = deque()
operator = deque()

last_dig = 0
find_nex = False
i = 0
last_dig_continue = False
while i < len(s):
if s[i] == " ":
i += 1
continue
if find_nex:
find_nex = False
while i < len(s) and s[i].isdigit():
last_dig = last_dig * 10 + int(s[i])
i += 1
op = operator.pop()
last = num.pop()
if op == "*":
num.append(last * last_dig)
else:
num.append(last // last_dig)
last_dig = 0
last_dig_continue = False
else:
if s[i].isdigit():
last_dig = last_dig * 10 + int(s[i])
last_dig_continue = True
elif s[i] == "+" or s[i] == "-":
if last_dig_continue:
num.append(last_dig)
last_dig = 0
operator.append(s[i])
elif s[i] == "*" or s[i] == "/":
if last_dig_continue:
num.append(last_dig)
last_dig = 0
find_nex = True
operator.append(s[i])
i += 1
if last_dig_continue: num.append(last_dig)
while num:
if not operator: return num.pop()
first, second = num.popleft(), num.popleft()
op = operator.popleft()
if op == "+":
num.appendleft(first + second)
else:
num.appendleft(first - second)
return ans
``````