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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
Note: Do not use the eval built-in library function.
解题思路：
只需要确定下每个括号后面需要带入的符号，用Stack即可，JAVA实现如下：
public int calculate(String s) {
Stack<Integer> sign = new Stack<Integer>();
sign.push(1);
int lastOp = 1;
int res = 0;
for (int i = 0; i < s.length(); i++) {
switch (s.charAt(i)) {
case ‘ ‘:
break;
case ‘+‘:
lastOp = 1;
break;
case ‘-‘:
lastOp = -1;
break;
case ‘(‘:
sign.push(lastOp*sign.peek());
lastOp=1;
break;
case ‘)‘:
sign.pop();
break;
default:
int num = 0;
while (i < s.length() && Character.isDigit(s.charAt(i)))
num = num * 10 + s.charAt(i++) - ‘0‘;
i--;
res += lastOp * num * sign.peek();
}
}
return res;
}
Java for LeetCode 224 Basic Calculator
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原文地址：http://www.cnblogs.com/tonyluis/p/4579092.html
