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  1. #include <iostream>
  2. #include <vector>
  3.  
  4. //Find Majority element i.e. the element that occurs more than 1/2 of the array
  5. // Worst Case Complexity :  O(n)
  6. template <typename DataType>
  7. DataType FindMajorityElement(std::vector<DataType> arr) {
  8.     // Modified BOYERS ALGORITHM with forward and reverse passes
  9.     // Count will stay positive if there is a majority element
  10.     auto GetMajority = [](auto seq_begin, auto seq_end) -> DataType{
  11.         int count = 1;
  12.         DataType majority = *(seq_begin);
  13.         for (auto itr = seq_begin+1; itr != seq_end; ++itr) {
  14.             count += (*itr == majority) ? 1 : -1;
  15.             if (count <= 0) {   // Flip the majority and set the count to zero whenever it falls below zero
  16.                 majority = *(itr);
  17.                 count = 0;
  18.             }
  19.         }
  20.         return majority;
  21.     };
  22.     DataType majority1 = GetMajority(arr.begin(), arr.end());
  23.     DataType majority2 = GetMajority(arr.rbegin(), arr.rend());
  24.     int maj1_count = 0, maj2_count = 0;
  25.     // Check if any of the the majority elements is really the majority
  26.     for (const auto& itr: arr) {
  27.         maj1_count += majority1 == itr ? 1 : 0;
  28.         maj2_count += majority2 == itr ? 1 : 0;
  29.     }
  30.     if (maj1_count >= arr.size()/2)
  31.         return majority1;
  32.     if (maj2_count >= arr.size()/2)
  33.         return majority2;
  34.     // else return -1
  35.     return -1;
  36. }
  37.  
  38. int main() {
  39.     using namespace std;
  40.     std::cout <<  FindMajorityElement(std::vector<int> { 1, 10, 2, 10, 1, 10, 4, 10, 5, 10 }) << endl;
  41.     std::cout <<  FindMajorityElement(std::vector<int> { 10, 1, 10, 2, 10, 1, 10, 4, 10, 5 }) << endl;
  42.     std::cout <<  FindMajorityElement(std::vector<int> { 1, 2, 1, 10, 10, 10, 10, 4, 10, 5 }) << endl;
  43.     std::cout <<  FindMajorityElement(std::vector<int> { 1, 1, 1, 10, 10, 10, 10, 10, 5, 4 }) << endl;
  44.     std::cout <<  FindMajorityElement(std::vector<float> { 1.0f, 1.0f, 1.0f, 10.f, 10.0f, 10.0f, 10, 10, 5, 4 }) << endl;
  45.     return 0;
  46. }
  47.  
  48.  
Success #stdin #stdout 0s 3464KB
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https://ideone.com/roCQKN
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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