<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) {
        stack<pair<int,char>>st;
        int sign = 1;
        long long ans = 0;
        for(int i=0;i<s.length();i++){
            if(isdigit(s[i])){
                long long num = 0;
                while(i < s.length() && isdigit(s[i])){
                    num = num*10 + (s[i] - '0');
                    i++;
                }
                i--;
                if(!st.empty() && (st.top().second == '*' || st.top().second == '/')){
                    if(st.top().second=='*'){
                        st.pop();
                        num*=st.top().first;
                        st.pop();
                    }
                    else{
                        st.pop();
                        num = st.top().first/num;
                        st.pop();
                    }
                }
                st.push({num,'+'});
            }

            else if(s[i] == '+')st.push({-1,'+'});
            else if(s[i] == '-')st.push({-1,'-'});
            else if(s[i] == '*')st.push({-1,'*'});
            else if(s[i] == '/')st.push({-1,'/'});
        }
        vector<int>vec;
        while(!st.empty()){
            if(st.top().first == -1){
                if(st.top().second == '-')vec.push_back(-2);
                else vec.push_back(-1);
            }
            else vec.push_back(st.top().first);
            st.pop();
        }
        reverse(vec.begin(),vec.end());
        if(vec.size())
        ans = vec[0];
        for(int i=1;i<vec.size();i++){
            if(vec[i] == -1){
                ans += vec[i+1];
            }
            else{
                ans -= vec[i+1];
            }
            i++;
        }
        return ans;
    }
};
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