<p>Given a string <code>s</code> which represents an expression, <em>evaluate this expression and return its value</em>.&nbsp;</p>

<p>The integer division should truncate toward zero.</p>

<p>You may assume that the given expression is always valid. All intermediate results will be in the range of <code>[-2<sup>31</sup>, 2<sup>31</sup> - 1]</code>.</p>

<p><strong>Note:</strong> You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as <code>eval()</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> s = "3+2*2"
<strong>Output:</strong> 7
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> s = " 3/2 "
<strong>Output:</strong> 1
</pre><p><strong class="example">Example 3:</strong></p>
<pre><strong>Input:</strong> s = " 3+5 / 2 "
<strong>Output:</strong> 5
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 3 * 10<sup>5</sup></code></li>
	<li><code>s</code> consists of integers and operators <code>(&#39;+&#39;, &#39;-&#39;, &#39;*&#39;, &#39;/&#39;)</code> separated by some number of spaces.</li>
	<li><code>s</code> represents <strong>a valid expression</strong>.</li>
	<li>All the integers in the expression are non-negative integers in the range <code>[0, 2<sup>31</sup> - 1]</code>.</li>
	<li>The answer is <strong>guaranteed</strong> to fit in a <strong>32-bit integer</strong>.</li>
</ul>

class Solution {
public:
    int calculate(string s) 
    {
        int n = s.size(); 
        int ret = 0; 
        int sum = 0; 
        int cur = 0;
        char sign = '+';

        for (int i = 0; i < n; ++i)
        {
            char c = s[i]; 
            if (c >= '0' && c <= '9')
            {
                cur = cur * 10 + (c - '0');
            }
            else if (c == '(')
            {
                int count = 0; 
                int j = 0; 
                for (; i < n; ++i)
                {
                    if (s[i] == '(') ++count; 
                    else if (s[i] == ')') --count; 
                    if (count == 0) break; 
                }

                cur = calculate(s.substr(j, i - j - 1));
            }

            if (c == '+' || c == '-' || c == '*' || c == '/' ||  i == n - 1)
            {
                switch (sign)
                {
                    case '+': sum += cur; break; 
                    case '-': sum -= cur; break; 
                    case '*': sum *= cur; break; 
                    case '/': sum /= cur; break; 
                }

                if (c == '+' || c == '-' || i == n - 1)
                {
                    ret += sum; 
                    sum = 0; 
                }

                cur = 0; 
                sign = c; 
            }
        }
        return ret; 
    }
};

//  2 * 3 + (4 + 5 * 9)
10820