<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 {
public:
    vector<vector<int>> merge(vector<vector<int>>& intervals) {
        // [1,3]
        // [2,6]
        // [1,6]

        // cache up the interval {start, end} to dataMap
        // use the map lower_bound -> find the interval >= start

        vector<vector<int>> res;
        map<int, int> dataMap;  // {start, end}

        for(auto &interval : intervals){
            int start = interval[0];
            int end = interval[1];

            // exception
            if(start > end){
                continue;
            }

            if(dataMap.size() == 0){
                dataMap[start] = end;
                continue;
            }

            //    [20, 30]
            //  [40, 50]   [10, 15]
            if(start > prev(dataMap.end())->second || end < dataMap.begin()->first){
                dataMap[start] = end;
                continue;
            }

            // general (overlap occurred)
            //   [20, 30]  prevIt
            //   [30, 40]    it
            //   [25, 35]  {start, end}

            auto it = dataMap.lower_bound(start);
            auto prevIt = prev(it);

            // merge
            if(it != dataMap.begin() && start <= prevIt->second){
                start = prevIt->first;
                end = max(end, prevIt->second);
                dataMap.erase(prevIt);
            }

            while(it != dataMap.end() && end >= it->first){
                end = max(end, it->second);
                it = dataMap.erase(it);
            }

            dataMap[start] = end;
        }

        for(auto &[start, end] : dataMap){
            res.push_back(vector<int>{start, end});
        }
        return res;
    }
};
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