224. Basic Calculator

224. Basic Calculator

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.

思路： 遇到 ‘(’ 就把之前的结果和符号push进stack. 遇到’)’就把当前结果*stack中的符号再加上stack中之前的结果.

class Solution { public int calculate(String s) { if(s==null || s.length() == 0) return 0; Stack stack = new Stack(); int res = 0; int sign = 1; for(int i=0; i

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