Largest Positive Integer That Exists With Its Negative
<p>Given an integer array <code>nums</code> that <strong>does not contain</strong> any zeros, find <strong>the largest positive</strong> integer <code>k</code> such that <code>-k</code> also exists in the array.</p>

<p>Return <em>the positive integer </em><code>k</code>. If there is no such integer, return <code>-1</code>.</p>

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

<pre>
<strong>Input:</strong> nums = [-1,2,-3,3]
<strong>Output:</strong> 3
<strong>Explanation:</strong> 3 is the only valid k we can find in the array.
</pre>

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

<pre>
<strong>Input:</strong> nums = [-1,10,6,7,-7,1]
<strong>Output:</strong> 7
<strong>Explanation:</strong> Both 1 and 7 have their corresponding negative values in the array. 7 has a larger value.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> nums = [-10,8,6,7,-2,-3]
<strong>Output:</strong> -1
<strong>Explanation:</strong> There is no a single valid k, we return -1.
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 1000</code></li>
	<li><code>-1000 &lt;= nums[i] &lt;= 1000</code></li>
	<li><code>nums[i] != 0</code></li>
</ul>
Array; Hash Table; Two Pointers; Sorting
class Solution {
    public int findMaxK(int[] nums) {
        int ans = -1;
        Set<Integer> s = new HashSet<>();
        for (int x : nums) {
            s.add(x);
        }
        for (int x : s) {
            if (s.contains(-x)) {
                ans = Math.max(ans, x);
            }
        }
        return ans;
    }
}