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.
Example
No.1
Input: “3+2*2”
Output: 7
No.2
Input: “ 3/2 “
Output: 1
No.3
Input: “ 3+5 / 2 “
Output: 5
Note
- You may assume that the given expression is always valid.
- Do not use the eval built-in library function.
Code
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public int (String s) {
int result = 0;
int curResult = 0;
int num = 0;
char lastSign = '+';
for (int i = 0; i < s.length(); i++) {
char ch = s.charAt(i);
if (Character.isDigit(ch)) {
num = 10 * num + (ch - '0');
}
if (ch == '+' || ch == '-' || ch == '*' || ch == '/' || i == s.length() - 1) {
switch (lastSign) {
case '+': curResult += num; break;
case '-': curResult -= num; break;
case '*': curResult *= num; break;
case '/': curResult /= num; break;
}
if (ch == '+' || ch == '-' || i == s.length() - 1) {
result += curResult;
curResult = 0;
}
lastSign = ch;
num = 0;
}
}
return result;
}
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