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

       string ans="";
       int i=num.size();
        while(!st.empty()){
            num[i-1]=st.top();
            // ans=st.top()+ans;
            st.pop();
            i--;
            
        }
       
        return num.substr(i);
    }
};
10100