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.

class Solution {public:    int calculate_help(int left, char symbol, int right)    {    switch(symbol)    {    case '*':    return left * right;    case '/':    return left / right;    default:    return 0;    }    }        int calculate(string s) {        int n = s.length();    if (n < 1)    {    return 0;    }    int result = 0;    stack<int> value;    stack<char> symbol;    symbol.push('+');    for (int i = 0; i < n;)    {    if (s[i] == '+' || s[i] == '-'    || s[i] == '*' || s[i] == '/')    {    symbol.push(s[i]);    i++;    }    else if (s[i] == ' ')    {        i++;    }    else    {    int right = 0;    while (i < n && s[i] <= '9' && s[i] >= '0')    {    right = right*10 + s[i]-'0';    i++;    }    if (!symbol.empty() && (symbol.top() == '*' || symbol.top() == '/'))    {    right = calculate_help(value.top(), symbol.top(), right);    value.pop();    symbol.pop();    }    value.push(right);    }    }    while (!symbol.empty())    {    int temp = value.top();    if (symbol.top() == '-')    {    temp = -temp;    }    result += temp;    value.pop();    symbol.pop();    }    return result;    }};