Find the Most Competitive Subsequence
<p>Given an integer array <code>nums</code> and a positive integer <code>k</code>, return <em>the most<strong> competitive</strong> subsequence of </em><code>nums</code> <em>of size </em><code>k</code>.</p>

<p>An array&#39;s subsequence is a resulting sequence obtained by erasing some (possibly zero) elements from the array.</p>

<p>We define that a subsequence <code>a</code> is more <strong>competitive</strong> than a subsequence <code>b</code> (of the same length) if in the first position where <code>a</code> and <code>b</code> differ, subsequence <code>a</code> has a number <strong>less</strong> than the corresponding number in <code>b</code>. For example, <code>[1,3,4]</code> is more competitive than <code>[1,3,5]</code> because the first position they differ is at the final number, and <code>4</code> is less than <code>5</code>.</p>

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

<pre>
<strong>Input:</strong> nums = [3,5,2,6], k = 2
<strong>Output:</strong> [2,6]
<strong>Explanation:</strong> Among the set of every possible subsequence: {[3,5], [3,2], [3,6], [5,2], [5,6], [2,6]}, [2,6] is the most competitive.
</pre>

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

<pre>
<strong>Input:</strong> nums = [2,4,3,3,5,4,9,6], k = 4
<strong>Output:</strong> [2,3,3,4]
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>0 &lt;= nums[i] &lt;= 10<sup>9</sup></code></li>
	<li><code>1 &lt;= k &lt;= nums.length</code></li>
</ul>
Stack; Greedy; Array; Monotonic Stack
class Solution {
public:
    vector<int> mostCompetitive(vector<int>& nums, int k) {
        vector<int> stk;
        int n = nums.size();
        for (int i = 0; i < n; ++i) {
            while (stk.size() && stk.back() > nums[i] && stk.size() + n - i > k) {
                stk.pop_back();
            }
            if (stk.size() < k) {
                stk.push_back(nums[i]);
            }
        }
        return stk;
    }
};