<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:

    vector<int> parse(string str){
        int num = 0;
        int n = str.length();

        bool zf = false;
        vector<int>nums;

        for(int i=0;i<n;i++){
            if(str[i] == ' ') continue;
            else if(str[i]>='0' && str[i]<='9'){
                num *= 10;
                num += str[i] - '0';
                if(str[i] == '0') zf = true;
                else zf = false;
            }
            else {
                nums.push_back(num);
                num = 0;
            }
        }
        if(num || zf) nums.push_back(num);
        return nums;
    }

    int calculate(string s) {
        vector<int> nums;
        int n = s.length();
        nums = parse(s);

        for(auto x: nums) cout<<x<<" ";
        cout<<endl;

        stack<int>st;
        st.push(nums[0]);
        int ind = 1;
        for(int i = 0;i<n;i++){
            if(s[i] == '+' || s[i] == '-') st.push(nums[ind++]);
            else if(s[i] == '*'){
                int a = st.top();st.pop();
                int b = nums[ind++];
                st.push(a * b);
            }
            else if(s[i] == '/'){
                int a = st.top();st.pop();
                int b = nums[ind++];
                st.push(a / b);
            }
        }

        // reverse stack.
        stack<int>st2;
        while(!st.empty()){
            int ele = st.top();st.pop();
            st2.push(ele);
        }

        for(int i =0;i<n;i++){
            if(s[i] == '-'){
                int a = st2.top();st2.pop();
                int b = st2.top();st2.pop();
                st2.push(a-b);
            }
            else if(s[i] == '+'){
                int a = st2.top();st2.pop();
                int b = st2.top();st2.pop();
                st2.push(a+b);
            }
        }

        return st2.top();
        // return -1;
    }
};
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