<p>Given an array&nbsp;of <code>intervals</code>&nbsp;where <code>intervals[i] = [start<sub>i</sub>, end<sub>i</sub>]</code>, merge all overlapping intervals, and return <em>an array of the non-overlapping intervals that cover all the intervals in the input</em>.</p>

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

<pre>
<strong>Input:</strong> intervals = [[1,3],[2,6],[8,10],[15,18]]
<strong>Output:</strong> [[1,6],[8,10],[15,18]]
<strong>Explanation:</strong> Since intervals [1,3] and [2,6] overlap, merge them into [1,6].
</pre>

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

<pre>
<strong>Input:</strong> intervals = [[1,4],[4,5]]
<strong>Output:</strong> [[1,5]]
<strong>Explanation:</strong> Intervals [1,4] and [4,5] are considered overlapping.
</pre>

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

<ul>
	<li><code>1 &lt;= intervals.length &lt;= 10<sup>4</sup></code></li>
	<li><code>intervals[i].length == 2</code></li>
	<li><code>0 &lt;= start<sub>i</sub> &lt;= end<sub>i</sub> &lt;= 10<sup>4</sup></code></li>
</ul>

class Solution final {
private:
    template<class C>
    static constexpr bool intersects(const C &first, const C &second) noexcept {
        return max(first[0], second[0]) <= min(first[1], second[1]);
    }

    template<class C>
    static constexpr void merge(C &first, const C &second) noexcept {
        first[0] = min(first[0], second[0]);
        first[1] = max(first[1], second[1]);
    }

public:
    constexpr vector<vector<int>> merge(
        vector<vector<int>> &intervals
    ) const noexcept {
        const auto last{intervals.end()};
        auto left{intervals.begin()};
        sort(left, last);

        for (auto right{next(left)}; right != last; ++right) {
            if (intersects(*left, *right))
                merge(*left, *right);
            else {
                ++left;
                if (left != right)
                    iter_swap(left, right);
            }
        }
        intervals.resize(size_t(distance(intervals.begin(), left)) + 1);

        return move(intervals);
    }
};

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