Java for LeetCode 224 Basic Calculator

標籤：

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.

解題思路：

只需要確定下每個括弧後面需要帶入的符號，用Stack即可，JAVA實現如下：

    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