<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 (num.size() == k) return "0";
        string ret = "";
        ret.push_back(num[0]);

        int i = 1;
        for (; i < num.size() && k; i++) {
            char curr = num[i];
            while (ret.size() && k && ret.back() > curr) {
                ret.pop_back();
                k--;
            }

            ret.push_back(curr);
        }

        if (i < num.size()) {
            if (i < num.size()) ret += num.substr(i);
        }

        if (k) {
            ret = ret.substr(0, ret.size() - k);
        }

        int j = 0;
        while (j < ret.size()) {
            if (ret[j] == '0') j++;
            else break;
        }

        if (j != 0) {
            ret = ret.substr(j);
        }

        if (ret.empty()) return "0";

        return ret;
    }
};
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