<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) {
        string s = "";        
        stack<int> st;
        long long i=0, n=num.length();

        while (i<n) {
            while (!st.empty() && k>0 && st.top() > (num[i]-'0'))  k--,st.pop();

            st.push(num[i]-'0');
            i++;
        }

        while (k--) st.pop();

        while (!st.empty()) {
            s.push_back(st.top()+'0');
            st.pop();
        }

        // n=s.length();
        // i=n-1;

        // while (i>=0 && s[i]=='0') {
        //     s.pop_back();
        //     i--;
        // }


        while (s.size()!=0 && s.back()=='0') {
            s.pop_back();
        }

        if(s.length()==0)   return "0";
        reverse(s.begin(), s.end());
        return s;
    }
};
12400