<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){
        s.push_back(' ');
    stack<int> st;
    int n = s.size();
    int currNum=-1;
    for(int i=0;i<n;i++){
        if((s[i]==' '|| s[i]=='+' || s[i]=='-' || s[i]=='*' || s[i]=='/') && st.size()>1 && currNum!=-1){
            if(st.top()==-1){
                st.pop();
                int num1=st.top();
                st.pop();
                st.push(num1*currNum);
            }else if(st.top()==-2){
                st.pop();
                int num1=st.top();
                st.pop();
                st.push(num1/currNum);
            }else if(currNum!=-1) st.push(currNum);
            currNum=-1;
        }
        if(s[i]=='*'){
            if(currNum!=-1) st.push(currNum);
            st.push(-1);
                currNum=-1;
        }else if(s[i]=='/'){
            if(currNum!=-1) st.push(currNum);
            st.push(-2);
            currNum=-1;
        }else if(s[i]=='+'){
            if(currNum!=-1) st.push(currNum);
            st.push(-3);
            currNum=-1;
        }else if(s[i]=='-'){
            if(currNum!=-1) st.push(currNum);
            st.push(-4);
                currNum=-1;
        }else if(s[i]>='0' && s[i]<='9'){
            if(currNum==-1){
                currNum = (s[i]-'0');
            }else{
                currNum*=10;
                currNum+=(s[i]-'0');
            }
        }
    }
    if(currNum!=-1) st.push(currNum);
    stack<int> tempStack;
    while (!st.empty()) {
        tempStack.push(st.top());
        st.pop();
    }
    st = tempStack;
    while(st.size()>2){
        int num2 = st.top();
        st.pop();
        int op = st.top();
        st.pop();
        int num1 = st.top();
        st.pop();
        if(op==-3){
            st.push(num1+num2);
        }else{
            st.push(num2-num1);
        }
    }
    return st.top();
}
};
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