<p>Given string num representing a non-negative integer <code>num</code>, and an integer <code>k</code>, return <em>the smallest possible integer after removing</em> <code>k</code> <em>digits from</em> <code>num</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> num = &quot;1432219&quot;, k = 3
<strong>Output:</strong> &quot;1219&quot;
<strong>Explanation:</strong> Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> num = &quot;10200&quot;, k = 1
<strong>Output:</strong> &quot;200&quot;
<strong>Explanation:</strong> Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> num = &quot;10&quot;, k = 2
<strong>Output:</strong> &quot;0&quot;
<strong>Explanation:</strong> Remove all the digits from the number and it is left with nothing which is 0.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= k &lt;= num.length &lt;= 10<sup>5</sup></code></li>
	<li><code>num</code> consists of only digits.</li>
	<li><code>num</code> does not have any leading zeros except for the zero itself.</li>
</ul>

class Solution {
public:
    string removeKdigits(string num, int k) {
        stack<int>st;
        int n=num.length();

        for(int i=0; i<n; i++){
            while(!st.empty() && k>0 && st.top()-'0'>num[i]-'0'){
                st.pop();
                k=k-1;
            }
            st.push(num[i]);
        }
        while(k>0){
            st.pop();
            k--;
            }
            if(st.empty())return "0";

            string res="";
            while(!st.empty()){
                res.push_back(st.top());
                st.pop();
            }
            while(res.size()!=0 && res.back()=='0'){
                res.pop_back();
            }
            if(res.empty())return "0";
            reverse(res.begin(),res.end());

            return res;
    }
};
12500