(medium)LeetCode 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.

代码如下：

public class Solution {
    public int calculate(String s) {
       Stack<Integer>stack=new Stack<Integer>();
       int ret=0;
       int num=0;
       int sign=1;
       for(int i=0;i<s.length();i++){
           char c=s.charAt(i);
           if(Character.isDigit(c)){
               num=10*num+c-'0';
           }
           else if(c=='+'){
               ret+=sign*num;
               num=0;
               sign=1;
           }else if(c=='-'){
               ret+=sign*num;
               num=0;
               sign=-1;
           }else if(c=='('){
               stack.push(ret);
               stack.push(sign);
               sign=1;
               ret=0;
           }else if(c==')'){
               ret+=sign*num;
               num=0;
               ret*=stack.pop();
               ret+=stack.pop();
           }
       }
       if(num!=0) ret+=sign*num;
       return ret;
    }
}

　　运行结果：