<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) {
        int n = num.size(), k1 = k;

        if (n==k) return "0";

        stack<char> stk;

        for(int i=0;i<n;i++) {
            while(!stk.empty() && k1>0) {
                if (stk.top() - '0' > num[i] - '0') {
                    stk.pop();
                    k1--;
                } else {
                    break;
                }
            }

            stk.push(num[i]);
        }

        while(!stk.empty() && k1>0) {
            stk.pop();
            k1--;
        }

        string ans="";
        while(!stk.empty()) {
            ans += stk.top();
            stk.pop();
        }

        while(ans.size() > 0 && ans.back() == '0')
        ans.pop_back();

        reverse(ans.begin(), ans.end());

        return ans.size() == 0 ? "0": ans;
    }
};
10500