<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<char> sign;
        stack<int> num;

        string n="";
        for(int i=0; i<s.size(); i++){
            if(s[i]!=' '){
                n.push_back(s[i]);
            }
        }
        s=n;
        int i=0;
        while(i<s.size()){
            if(s[i]=='+' || s[i]=='-' || s[i]=='*' || s[i]=='/'){
                if(s[i]=='/'){
                    int x=num.top();
                    num.pop();
                    string tmp="";
                    i++;
                    while(s[i]>='0' && s[i]<='9'){
                        tmp.push_back(s[i]);
                        i++;
                    }
                    i--;
                    int y=stoi(tmp);
                    int res=x/y;

                    num.push(res);
                }
                else if(s[i]=='*'){
                    int x=num.top();
                    num.pop();
                    string tmp="";
                    i++;
                    while(s[i]>='0' && s[i]<='9'){
                        tmp.push_back(s[i]);
                        i++;
                    }
                    i--;
                    int y=stoi(tmp);
                    int res=x*y;

                    num.push(res);
                }
                else{
                    sign.push(s[i]);
                }
            }
            else if(s[i]>='0' && s[i]<='9'){
                string ans="";
                while(s[i]<='9' && s[i]>='0'){
                    ans.push_back(s[i]);
                    i++;
                }
                i--;
                num.push(stoi(ans));
            }
            i++;
        }
        
        stack<int> number;
        while(!sign.empty()){
            if(sign.top()=='-'){
                number.push(num.top()*(-1));
            }

            else{
                number.push(num.top());
            }
            num.pop();
            sign.pop();
        }

        if(!num.empty()){
            number.push(num.top());
        }
        int ans=0;

        while(!number.empty()){
            ans=ans+number.top();
            number.pop();
        }

        return ans;
    }
};
18681