<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) {
        for(int i = 0; i < s.size(); ++i){
            if(s[i] == '*'){
                string num1;
                string num2;
                int tmp1 = i-1;
                while(tmp1 >= 0 && s[tmp1] != '*' && s[tmp1] != '+' && s[tmp1] != '-' && s[tmp1] != '/'){
                    num1.insert(num1.begin(), s[tmp1]);
                    s.erase(s.begin()+tmp1);
                    i--;
                    tmp1--;
                }
                int tmp2 = i+1;
                while(tmp2 < s.size() && s[tmp2] != '*' && s[tmp2] != '+' && s[tmp2] != '-' && s[tmp2] != '/'){
                    num2.push_back(s[tmp2]);
                    s.erase(s.begin()+tmp2);
                }
                int prod = stoi(num1)*stoi(num2);
                string firstHalf = s.substr(0, i);
                string secondHalf = s.substr(i+1, s.size()-1);
                s = firstHalf + to_string(prod) + secondHalf;
            }else if(s[i] == '/'){
                string num1;
                string num2;
                int tmp1 = i-1;
                while(tmp1 >= 0 && s[tmp1] != '*' && s[tmp1] != '+' && s[tmp1] != '-' && s[tmp1] != '/'){
                    num1.insert(num1.begin(), s[tmp1]);
                    s.erase(s.begin()+tmp1);
                    i--;
                    tmp1--;
                }
                int tmp2 = i+1;
                while(tmp2 < s.size() && s[tmp2] != '*' && s[tmp2] != '+' && s[tmp2] != '-' && s[tmp2] != '/'){
                    num2.push_back(s[tmp2]);
                    s.erase(s.begin()+tmp2);
                }
                int prod = stoi(num1)/stoi(num2);
                string firstHalf = s.substr(0, i);
                string secondHalf = s.substr(i+1, s.size()-1);
                s = firstHalf + to_string(prod) + secondHalf;
            }
        }

        long long ans = 0;
        int sum = 1;
        int minus = 0;
        string tmp = "";
        for(int i = 0; i < s.size(); ++i){
            if(s[i] != '+' && s[i] != '-'){
                tmp += s[i];
            }
            if(s[i] == '+' && sum == 1){
                ans += stoi(tmp);
                tmp = "";
                sum = 1;
                minus = 0;
            }else if(s[i] == '+' && minus == 1){
                ans -= stoi(tmp);
                tmp = "";
                sum = 1;
                minus = 0;
            }else if(s[i] == '-' && sum == 1){
                ans += stoi(tmp);
                tmp = "";
                sum = 0;
                minus = 1;
            }else if(s[i] == '-' && minus == 1){
                ans -= stoi(tmp);
                tmp = "";
                sum = 0;
                minus = 1;
            }
            if(i == s.size()-1){
                if(sum == 1){
                    ans += stoi(tmp);
                }else{
                    ans -= stoi(tmp);
                }
            }
        }
        return ans;
    }
};
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