<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 {

    int solve(string &s, int l, int r)
    {
        if (r<l)
            return 0;

        for (int i=r;i>=l;i--)
        {
            if (s[i] == '-')
            {
                return solve(s, l, i-1) - solve(s, i+1, r);
            }
        
            if (s[i] == '+')
            {
                return solve(s, l, i-1) + solve(s, i+1, r);
            }
        }

        for (int i=r;i>=l;i--)
        {
            if (s[i] == '*')
            {
                return solve(s, l, i-1) * solve(s, i+1, r);
            }

            if (s[i] == '/')
            {
                return solve(s, l, i-1) / solve(s, i+1, r);
            }
        }

        int num = 0;
        for (int i=l;i<=r;i++){
            if (s[i] == ' ')    
                continue;
            num = num*10 + (s[i]-'0');
        }

        return num;
    }

public:
    int calculate(string s) {
        /*
            * / %
            + -
        */

        return solve(s, 0, s.length() - 1);
    }

    /*
    N is number of ops
    tc - O(N*N)
    sc - O(N)
    */  
};
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