feat(line-sweep,prefix): Add 1854

euchangxian · Dec 18, 2024 · f92f5f1 · f92f5f1
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diff --git a/C++/1854-MaximumPopulationYear/Solution.cpp b/C++/1854-MaximumPopulationYear/Solution.cpp
@@ -0,0 +1,41 @@
+#include <cstddef>
+#include <cstdlib>
+#include <map>
+#include <vector>
+
+class Solution {
+ public:
+  int maximumPopulation(std::vector<std::vector<int>>& logs) {
+    // logs[i] = [birth, death], birth/death year of the ith person.
+    // population is the number of people alive during that year.
+    // i-th person is counted in year x population if x is in the inclusive
+    // range [birth, death-1],
+    // Want: earliest year with the maximum population.
+    // Line Sweep.
+    // Events are the birth/death years. If birth, add 1 to the prefix sum,
+    // if death, -1. Take the max of the sum.
+
+    // {year, count}
+    std::map<int, int> events;
+    for (const auto& log : logs) {
+      int birth = log[0];
+      int death = log[1];
+
+      ++events[birth];
+      --events[death];
+    }
+
+    int maxPop = 0;
+    int maxYear = 0;
+
+    int pop = 0;
+    for (const auto [year, count] : events) {
+      pop += count;
+      if (pop > maxPop) {
+        maxPop = pop;
+        maxYear = year;
+      }
+    }
+    return maxYear;
+  }
+};
diff --git a/README.md b/README.md
@@ -429,6 +429,7 @@ Now solving in C++. Like it.
 | 1823 | FindTheWinnerOfTheCircularGame                                   |                                            [![C++](assets/c++.svg)](C++/1823-FindTheWinnerOfTheCircularGame/Solution.cpp)                                             |
 | 1829 | MaximumXORForEachQuery                                           |                                                [![C++](assets/c++.svg)](C++/1829-MaximumXORForEachQuery/Solution.cpp)                                                 |
 | 1845 | SeatReservationManager                                           |                                                [![C++](assets/c++.svg)](C++/1845-SeatReservationManager/Solution.cpp)                                                 |
+| 1854 | MaximumPopulationYear                                            |                                                 [![C++](assets/c++.svg)](C++/1854-MaximumPopulationYear/Solution.cpp)                                                 |
 | 1857 | LargestColorValueInADirectedGraph                                |                                           [![C++](assets/c++.svg)](C++/1857-LargestColorValueInADirectedGraph/Solution.cpp)                                           |
 | 1861 | RotatingTheBox                                                   |                                                    [![C++](assets/c++.svg)](C++/1861-RotatingTheBox/Solution.cpp)                                                     |
 | 1863 | SumOfAllSubsetXORTotals                                          |                                                [![C++](assets/c++.svg)](C++/1863-SumOfAllSubsetXORTotals/Solution.cpp)                                                |