Title description:
Give you a string expression s, please implement a basic calculator to calculate and return its value.
Integer division only retains the integer part.
prompt:
- 1 <= s.length <= 3 * 105
- s is composed of integers and operators ('+','-','*','/'), separated by some spaces
- s represents a valid expression, all integers in the expression are non-negative integers and are in the range [0, 231-1]
- The question data guarantees that the answer is a 32-bit integer
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
code show as below:
class Solution {
public int calculate(String str) {
String s = str.trim();
int n = s.length();
Stack<Integer> stack = new Stack<>();
int num = 0;
char sign = '+';
for (int i = 0; i < n; i++) {
char ch = s.charAt(i);
if (Character.isSpaceChar(ch)) {
continue;
}
if (Character.isDigit(ch)) {
num = 10 * num + (ch - '0');
}
if (!Character.isDigit(ch) || i == n - 1) {
if (sign == '+') {
stack.push(num);
} else if (sign == '-') {
stack.push(-num);
} else if (sign == '*') {
stack.push(stack.pop() * num);
} else if (sign == '/') {
if (num != 0) {
stack.push((int) (stack.pop() / num));
}
}
sign = ch;
num = 0;
}
}
int ans = 0;
while (!stack.isEmpty()) {
ans += stack.pop();
}
return ans;
}
}
Results of the:
