(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-negati ve integers and empty spaces .

Assume that the given expression was 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.

The code is as follows:

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;    }}

Operation Result:

(medium) Leetcode 224.Basic Calculator