<p>A city&#39;s <strong>skyline</strong> is the outer contour of the silhouette formed by all the buildings in that city when viewed from a distance. Given the locations and heights of all the buildings, return <em>the <strong>skyline</strong> formed by these buildings collectively</em>.</p>

<p>The geometric information of each building is given in the array <code>buildings</code> where <code>buildings[i] = [left<sub>i</sub>, right<sub>i</sub>, height<sub>i</sub>]</code>:</p>

<ul>
	<li><code>left<sub>i</sub></code> is the x coordinate of the left edge of the <code>i<sup>th</sup></code> building.</li>
	<li><code>right<sub>i</sub></code> is the x coordinate of the right edge of the <code>i<sup>th</sup></code> building.</li>
	<li><code>height<sub>i</sub></code> is the height of the <code>i<sup>th</sup></code> building.</li>
</ul>

<p>You may assume all buildings are perfect rectangles grounded on an absolutely flat surface at height <code>0</code>.</p>

<p>The <strong>skyline</strong> should be represented as a list of &quot;key points&quot; <strong>sorted by their x-coordinate</strong> in the form <code>[[x<sub>1</sub>,y<sub>1</sub>],[x<sub>2</sub>,y<sub>2</sub>],...]</code>. Each key point is the left endpoint of some horizontal segment in the skyline except the last point in the list, which always has a y-coordinate <code>0</code> and is used to mark the skyline&#39;s termination where the rightmost building ends. Any ground between the leftmost and rightmost buildings should be part of the skyline&#39;s contour.</p>

<p><b>Note:</b> There must be no consecutive horizontal lines of equal height in the output skyline. For instance, <code>[...,[2 3],[4 5],[7 5],[11 5],[12 7],...]</code> is not acceptable; the three lines of height 5 should be merged into one in the final output as such: <code>[...,[2 3],[4 5],[12 7],...]</code></p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/12/01/merged.jpg" style="width: 800px; height: 331px;" />
<pre>
<strong>Input:</strong> buildings = [[2,9,10],[3,7,15],[5,12,12],[15,20,10],[19,24,8]]
<strong>Output:</strong> [[2,10],[3,15],[7,12],[12,0],[15,10],[20,8],[24,0]]
<strong>Explanation:</strong>
Figure A shows the buildings of the input.
Figure B shows the skyline formed by those buildings. The red points in figure B represent the key points in the output list.
</pre>

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

<pre>
<strong>Input:</strong> buildings = [[0,2,3],[2,5,3]]
<strong>Output:</strong> [[0,3],[5,0]]
</pre>

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

<ul>
	<li><code>1 &lt;= buildings.length &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= left<sub>i</sub> &lt; right<sub>i</sub> &lt;= 2<sup>31</sup> - 1</code></li>
	<li><code>1 &lt;= height<sub>i</sub> &lt;= 2<sup>31</sup> - 1</code></li>
	<li><code>buildings</code> is sorted by <code>left<sub>i</sub></code> in&nbsp;non-decreasing order.</li>
</ul>

class Solution {
public:
    bool sortcol(const vector<int>& v1, const vector<int>& v2) {
        return v1[2] > v2[2];
    }
    vector<vector<int>> find_sky(vector<vector<int>>& buildings) {
        vector<vector<int>> values;
        map<int, int> val;
        set<int> unique_nos;
        int i, j, k, n, m, ij, il, h, k1;
        n = buildings.size();
        for (i = 0; i < n; i++) {
            unique_nos.insert(buildings[i][0]);
            unique_nos.insert(buildings[i][1]);
        }
        for (auto i = unique_nos.begin(); i != unique_nos.end(); i++) {
            if (i == unique_nos.begin()) {
                continue;
            } else {
                unique_nos.insert(*i - 1);
            }
        }
        sort(buildings.begin(), buildings.end(),
             [this](vector<int> a, vector<int> b) { return sortcol(a, b); });
        for (i = 0; i < n; i++) {
            auto it1 = unique_nos.end();
            auto it2 = unique_nos.end();
            set<int> tobedeletedNos;
            for (auto ip = unique_nos.begin(); ip != unique_nos.end(); ip++) {
                if (*ip >= buildings[i][0] && *ip <= buildings[i][1] &&
                    it1 == unique_nos.end()) {
                    it1 = ip;
                    it2 = ip;
                }
                if (*ip <= buildings[i][1] && it1 != unique_nos.end()) {
                    it2 = ip;
                }
            }
            if (it1 != unique_nos.end()) {
                h = buildings[i][2];
                while (1) {
                    k = *it1;
                    tobedeletedNos.insert(k);
                    if (val.find(k) != val.end()) {
                        if (val[k] > h) {
                            h = val[k];
                        }
                        val.find(k)->second = h;
                    } else {
                        val.insert(pair<int, int>(k, h));
                    }
                    if (it1 == it2) {
                        break;
                    }
                    it1++;
                }
                for (auto i1 = tobedeletedNos.begin();
                     i1 != tobedeletedNos.end(); i1++) {
                    unique_nos.erase(*i1);
                }
            }
        }
        n = val.size();
        k = 0;
        for (auto itr = val.begin(); itr != val.end(); itr++) {
            if (itr == val.begin()) {
                k++;
                values.push_back({itr->first, itr->second});
            } else {
                auto itr1 = itr;
                auto itr2 = itr;
                --itr2;
                ++itr1;
                if (itr1 == val.end()) {
                    k++;
                    values.push_back({itr->first, 0});
                } else {
                    if (!(itr1->second >= itr->second &&
                          itr2->second >= itr->second)) {
                        if (values[k - 1][1] == itr->second) {
                            values.push_back({itr->first, itr1->second});
                        } else {
                            values.push_back({itr->first, itr->second});
                        }
                        k++;
                    }
                }
            }
        }
        return values;
    }
    vector<vector<int>> getSkyline(vector<vector<int>>& buildings) {
        vector<vector<int>> values;
        int i, j, k, m, n, l, r, h, il, maxil;
        n = buildings.size();
        il = 0;
        maxil = buildings[0][1];
        for (i = 1; i < n; i++) {
            if (buildings[i][0] > maxil) {
                vector<vector<int>> values1;
                vector<vector<int>> buildings1;
                for (j = il; j < i; j++) {
                    buildings1.push_back(
                        {buildings[j][0], buildings[j][1], buildings[j][2]});
                }
                il = i;
                values1 = find_sky(buildings1);
                values.insert(values.end(), values1.begin(), values1.end());
            }
            if (buildings[i][1] > maxil) {
                maxil = buildings[i][1];
            }
        }
        vector<vector<int>> values1;
        vector<vector<int>> buildings1;
        for (j = il; j < i; j++) {
            buildings1.push_back(
                {buildings[j][0], buildings[j][1], buildings[j][2]});
        }
        if (buildings1.size() > 0) {
            values1 = find_sky(buildings1);
            values.insert(values.end(), values1.begin(), values1.end());
        }
        return values;
    }
};
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