Question

Given an array of n integers, where one element appears more than n/2 times. We need to find that element in linear time and constant extra space.

YAAQ: Yet another arrays question.

Answer 1

I have a sneaking suspicion it’s something along the lines of (in C#)

// We don't need an array public int FindMostFrequentElement(IEnumerable<int> sequence) {     // Initial value is irrelevant if sequence is non-empty,     // but keeps compiler happy.     int best = 0;      int count = 0;      foreach (int element in sequence)     {         if (count == 0)         {             best = element;             count = 1;         }         else         {             // Vote current choice up or down             count += (best == element) ? 1 : -1;         }     }     return best; } 

It sounds unlikely to work, but it does. (Proof as a postscript file, courtesy of Boyer/Moore.)