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BasicCalculator.java
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BasicCalculator.java
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import java.util.ArrayList;
import java.util.List;
public class BasicCalculator {
/**
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 1:
Input: "3+2*2"
Output: 7
Example 2:
Input: " 3/2 "
Output: 1
Example 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.
*/
// public int calculate(String s) {
// if (s == null) return 0;
// s = s.replaceAll("\\s+", "");
// Stack<Integer> stack = new Stack<>();
// char sign = '+';
// int len = s.length();
// for (int i = 0; i < len; i++) {
// int num = 0;
// while (i < len && Character.isDigit(s.charAt(i))) { //while next char is also digit
// num *= 10;
// num += s.charAt(i) - '0';
// i++;
// }
// if (sign == '+') {
// stack.push(num);
// } else if (sign == '-') {
// stack.push(-num);
// } else if (sign == '*') {
// stack.push(stack.pop() * num);
// } else if (sign == '/') {
// stack.push(stack.pop() / num);
// }
// if (i < len)
// sign = s.charAt(i);
// }
// int res = 0;
// while (!stack.isEmpty()) {
// res += stack.pop();
// }
// return res;
// }
// 3+2*2
//tmp 3
//cur 2
//res 0
// nextSign *
//sign +
// i 3
public int calculate(String s) {
if (s == null || s.length() == 0) return 0;
s = s.replaceAll("\\s+", "");
int res = 0, tmp = 0;
char sign = '+';
for (int i = 0; i < s.length(); i++) {
int cur = 0;
while (i < s.length() && Character.isDigit(s.charAt(i))) {
cur = cur * 10 + (s.charAt(i) - '0');
i++;
}
if (sign == '*') {
tmp = tmp * cur;
} else if (sign == '/') {
tmp = tmp / cur;
} else if (sign == '+') {
res += tmp;
tmp = cur;
} else if (sign == '-') {
res += tmp;
tmp = -cur;
}
if (i < s.length())
sign = s.charAt(i);
}
res += tmp;
return res;
}
public static void main(String[] args) {
System.out.println("abc".startsWith("a"));
BasicCalculator a = new BasicCalculator();
System.out.println(a.calculate(" 3+5 / 2 "));
}
}