Array of size n, with one element n/2 times

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 arra

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 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.)