<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:
    
    void help1(int num,int &ans)
    {
         ans=ans+num;
        return;
    }
    
    void help2(int num,int &ans)
    {
        ans=ans-num;
        return;
    }
    
    int calculate(string s) 
    {
        vector<int>v1;
        int i,l=s.length();
        string temp="";
        for(i=0;i<l;i++)
        {
            if(s[i]!='+' && s[i]!='-' && s[i]!='*' && s[i]!='/')
            {
                temp.push_back(s[i]);
            }
            else
            {
                v1.push_back(stoi(temp,nullptr,10));
                temp="";
                if(s[i]=='+')
                {
                    v1.push_back(-1);
                }
                else if(s[i]=='-')
                {
                    v1.push_back(-2);
                }
                else if(s[i]=='*')
                {
                    v1.push_back(-3);
                }
                else
                {
                    v1.push_back(-4);
                }
            }
        }
        
        v1.push_back(stoi(temp,nullptr,10));
        
        vector<int>v2;
        for(i=0;i<v1.size();)
        {
            if(v1[i]!=-3 && v1[i]!=-4)
            {
                v2.push_back(v1[i]);
                i++;
            }
            else
            {
                int ms1=v2[v2.size()-1];
                v2.pop_back();
                int ms2=v1[i+1];
                if(v1[i]==-3)
                {
                    v2.push_back(ms1*ms2);
                }
                else if(v1[i]==-4)
                {
                    v2.push_back(ms1/ms2);
                }
                i=i+2;
             }
        }
        
        if(v2.size()==1)
        {
            return v2[0];
        }
        
        int ans=v2[0];
        for(i=1;i<v2.size(); )
        {
            if(v2[i]==-1)
            help1(v2[i+1],ans);
            else
            help2(v2[i+1],ans);
            i=i+2;    
        }
        return ans;
        
        
    }
};
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