[leetcode]Basic Calculator(java)

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问题描述：
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.

通用算法是使用栈结构+后缀表达式（如代码1），但是看到csdn的一篇博客突然觉得这样好傻。因为这里面只有+和-，那么完全可以将（）删除（代码2）。
这里面的思路是：使用一个栈，存放每个当前元素的系数（+1、-1），因为转变符号只有是-才会发生。

代码1：

 public int calculate(String s) {         Stack<Character> charStack = new Stack<Character>();         Stack<Integer> intStack = new Stack<Integer>();         int index = -1;         int length = s.length();         char tmpChar;         int top1,top2;         while((++index)<length){             if((tmpChar=s.charAt(index)) == ' ')//ignore blank                 continue;             switch (tmpChar) {                case '(':                    charStack.push(tmpChar);                    break;                case '-':                case '+':                    if(charStack.isEmpty())                        charStack.push(tmpChar);                    else if(charStack.peek()=='(')                        charStack.push(tmpChar);                    else {                        char op = charStack.pop();                        charStack.push(tmpChar);                        switch (op) {                            case '+':                                top1 = intStack.pop();                                top2 = intStack.pop();                                intStack.push(top1+top2);                                break;                            default:                                top1 = intStack.pop();                                top2 = intStack.pop();                                intStack.push(top2-top1);                                break;                        }                    }                    break;                case ')':                    while((tmpChar=charStack.pop())!='('){                        switch (tmpChar) {                        case '+':                            top1 = intStack.pop();                            top2 = intStack.pop();                            intStack.push(top1+top2);                            break;                        case '-':                            top1 = intStack.pop();                            top2 = intStack.pop();                            intStack.push(top2-top1);                            break;                        }                    }                    break;                default://number                    //form intnumber,and push to intstack                    int number = 0;                    while(tmpChar>='0'&&tmpChar<='9'){                        number = number*10+tmpChar-'0';                        if(++index<length)                            tmpChar = s.charAt(index);                        else                            break;                    }                    intStack.push(number);                    index--;                    break;                }         }         //process the last one         while(!charStack.isEmpty()){             tmpChar = charStack.pop();             switch (tmpChar) {                case '+':                    top1 = intStack.pop();                    top2 = intStack.pop();                    intStack.push(top2+top1);                    break;//+                default:                    top1 = intStack.pop();                    top2 = intStack.pop();                    intStack.push(top2-top1);                    break;//-            }         }         return (intStack.isEmpty())?0:intStack.pop();      }

代码2：

 public int calculate(String s){         int length = s.length();         Stack<Integer> signStack  = new Stack<Integer>();         signStack.push(1);//object         signStack.push(1);//firstElement sign         int index = -1;         char tmpChar;         int result = 0;         while(++index<length){             if((tmpChar=s.charAt(index))==' ')                continue;//ignore blank             switch (tmpChar) {            case '+':            case '(':                signStack.push(signStack.peek());                break;            case '-':                signStack.push(-1*(signStack.peek()));                break;            case ')':                signStack.pop();                break;            default://digital                int num=0;                while(tmpChar>='0'&&tmpChar<='9'){                    num=num*10+tmpChar-'0';                    if(++index<length)                        tmpChar = s.charAt(index);                    else                        break;                }                result+=signStack.pop()*num;                index--;                break;            }         }         return result;     }

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