<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 calcStack(string s) {

        stack<int> stk;

        char last_operation{'+'};
        int current_num{0};
        char current_char{'0'};

        for(int i = 0; i < s.length(); i++) {
            current_char = s[i];
            if(isdigit(current_char)) {
                current_num = current_num*10 + ((int)current_char - '0');
            }
            bool is_space = current_char == ' ';
            bool last_digit = i == s.length() - 1;

            // If we are the last digit or an operator, the same thing happens
            if(last_digit || !isdigit(current_char) && !is_space) {
                switch(last_operation) {
                    case '+':
                        stk.push(current_num);
                    break;
                    case '-':
                        stk.push(current_num * -1);
                    break;
                    case '*':
                        {
                            int last_val = stk.top();
                            stk.pop();
                            last_val = last_val * current_num;
                            stk.push(last_val);
                        }                    
                        break;
                    case '/':
                        {
                            int last_val = stk.top();
                            stk.pop();
                            last_val = last_val / current_num;
                            stk.push(last_val);
                        }                    
                    break;
                }

                current_num = 0;
                last_operation = current_char;

            }


        }
        
            int rval{0};
            while(!stk.empty()) {
                rval += stk.top();
                stk.pop();
            }

            return rval;
    }

int calcSavedNum(string s) {

        int rval{0};

        int last_number{0};
        char last_operation{'+'};
        int current_num{0};
        char current_char{'0'};

        for(int i = 0; i < s.length(); i++) {
            current_char = s[i];
            if(isdigit(current_char)) {
                current_num = current_num*10 + ((int)current_char - '0');
            }
            bool is_space = current_char == ' ';
            bool last_digit = i == s.length() - 1;

            // If we are the last digit or an operator, the same thing happens
            if(last_digit || !isdigit(current_char) && !is_space) {
                switch(last_operation) {
                    case '+':
                        rval += last_number;
                        last_number = current_num;
                    break;
                    case '-':
                        rval += last_number;
                        last_number = -1*current_num;
                    break;
                    case '*':
                        last_number *= current_num;
                        break;
                    case '/':
                        last_number /= current_num;
                    break;
                }

                current_num = 0;
                last_operation = current_char;

            }


        }
        rval += last_number;
        return rval;
    }



    int calculate(string s) {
        return calcSavedNum(s);
    }
};
11648