LeetCode 227. Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces. The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7" 3/2 " = 1" 3+5 / 2 " = 5

Note: Do not use the eval built-in library function.

This is one of the most tedious coding problem.

bool isOPTR(char s){        if(s == '+' || s == '-' || s == '*' || s == '/' || s == '\0' || s == '#') return true;        else return false;    }        char getPriority(char a, char b){        int i, j;        char priority[][5] = {            {'>', '>', '<', '<', '>'},            {'>', '>', '<', '<', '>'},            {'>', '>', '>', '>', '>'},            {'>', '>', '>', '>', '>'},            {'<', '<', '<', '<', '='}};   // it is pretty cool here to make a table.<img alt="得意" src="http://static.blog.csdn.net/xheditor/xheditor_emot/default/proud.gif" />        switch(a){            case '+': i = 0; break;            case '-': i = 1; break;            case '*': i = 2; break;            case '/': i = 3; break;            case '#': i = 4; break;        }        switch(b){            case '+': j = 0; break;            case '-': j = 1; break;            case '*': j = 2; break;            case '/': j = 3; break;            case '\0': j = 4; break;        }        return priority[i][j];    }        int Operation(int a, char theta, int b){        if(theta == '+') return (a + b);        else if(theta == '-') return (a - b);        else if(theta == '*') return (a * b);        else return (a / b);    }        int calculate(string s) {        int len = s.size();        if(len == 0) return 0;        int i = 0;                stack<char> OPTR;        OPTR.push('#');                stack<int> OPND;                while(s[i] != '\0' || OPTR.top()!= '#'){            if(s[i] == ' '){i++; continue;}            else if(!isOPTR(s[i])){                int sum = int(s[i] - '0');                int j = i + 1;                                while(s[j] != '\0' && s[j] != ' ' && !isOPTR(s[j])){                    sum = sum * 10 + int(s[j] - '0');                    j++;                }                OPND.push(sum);                i = j;                continue;            }else{                switch(getPriority(OPTR.top(), s[i])){                    case '<':                        OPTR.push(s[i]);                        i++;                        break;                    case '=':                        OPTR.pop();   // this one is actually not very necessary.                        i++;                        break;                    case '>':                        char theta = OPTR.top();                        OPTR.pop();                                                int b = OPND.top();                        OPND.pop();                                                int a = OPND.top();                        OPND.pop();                                                OPND.push(Operation(a, theta, b));                        break;                } // switch            } // else        } // while        return OPND.top();    }