SkylineProblem Solved Using Multiset and Sorting

leetcode/SkylineProblem.cpp

@@ -0,0 +1,34 @@
+// 218. The Skyline Problem 
+// Ques Topic : Ordered Set, Hashmaps and Heaps, Sorting.
+
+#include <bits/stdc++.h>
+using namespace std;
+class Solution {
+public:
+    vector<vector<int>> getSkyline(vector<vector<int>>& buildings) {
+        // use walls to record buildings; left x is an insertion event, and right x is a deletion event
+        vector<pair<int, int>>xaxis; // first: x, second: height
+        vector<vector<int>>ans;                 
+        for (auto b : buildings) {
+            // let left x has negative height to ensure left x goes to multiset first if with same 'x' as right x // add before removing is imp in case of same right x of one building and left x of other building
+            xaxis.push_back(make_pair(b[0], -b[2]));
+            xaxis.push_back(make_pair(b[1], b[2]));
+        }
+        sort(xaxis.begin(), xaxis.end());                   // sort walls
+        multiset<int>leftxHeights = {0};// keep left x heights sorted; dummy'0'for convenience
+        int mhabtpi = 0;     // current max height among leftxHeights
+        for (auto xh : xaxis) {
+            if (xh.second < 0) {   // it's a left x, insert the height
+                leftxHeights.insert(-xh.second);
+            } 
+            else {   // it's a right x, delete the height
+                leftxHeights.erase(leftxHeights.find(xh.second));
+            }
+
+            if (*leftxHeights.rbegin() != mhabtpi) {  // mark a skyline point
+                ans.push_back({xh.first,mhabtpi = *leftxHeights.rbegin()});
+            }
+        }
+        return ans; 
+    }
+};