<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 i=0;
        stack<int>st;
        stack<char>op;
        while(i<s.length())
        {
            if(s[i]>='0'&&s[i]<='9'){
            string num="";
            while(i<s.length()&&s[i]>='0'&&s[i]<='9')
            {
                num+=s[i];
                i++;
            }
            int k=stoi(num);
            st.push(k);
            }else if(s[i]=='+')
            {
                op.push(s[i]);
                i++;
            }else if(s[i]=='-')
            {
                op.push(s[i]);
                i++;
            }else if(s[i]=='*'){
                int k=st.top();
                st.pop();
                i++;
                if(s[i]>='0'&&s[i]<='9'){
                    string num="";
                    while(i<s.length()&&s[i]>='0'&&s[i]<='9')
                    {
                        num+=s[i];
                        i++;
                    }
                    int r=stoi(num);
                k=k*r;
                st.push(k);
                }
            
            }else if(s[i]=='/'){
                int k=st.top();
                st.pop();
                i++;
                if(s[i]>='0'&&s[i]<='9'){
                    string num="";
                    while(i<s.length()&&s[i]>='0'&&s[i]<='9')
                    {
                        num+=s[i];
                        i++;
                    }
                    int r=stoi(num);
                k=k/r;
                st.push(k);
                }
            }else{
                i++;
            }
        }
        stack<int>posi;
        stack<int>negi;
        while(!op.empty()&&st.size()>=2)
        {
            if(op.top()=='+')
            {
                posi.push(st.top());
                st.pop();
            }else{
                negi.push(st.top());
                st.pop();
            }
            op.pop();
        }
        int ans=0;
        while(!st.empty())
        {
            ans+=st.top();
            st.pop();
        }
        while(!posi.empty())
        {
            ans+=posi.top();
            posi.pop();
        }
        while(!negi.empty())
        {
            ans-=negi.top();
            negi.pop();
        }
        return ans;
    }
};
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