基本思想：如果将数组进行排序，那么出现次数超过一半的元素一定是中间的数，也就是中位数。我们利用快速排序的partition算法思想，寻找数组中位数。注意程序还要最后检查数组中是否真的有元素出现次数超过一半。

#include <iostream>
using namespace std;

struct BiTreeNode{
  int val;
  BiTreeNode* leftChild;
  BiTreeNode* rightChild;
};

int partition(int* arr, int length, int low, int high){
  if (arr == NULL || length <= 0 || low < 0 || high >= length)
    throw new exception("Invalid input data!");
  int pos = low + (high - low) / 2;
  int pivot = arr[pos];
  swap(arr[pos], arr[high]);
  int small = low - 1;
  for (int i = low; i < high; ++i){
    if (arr[i] < pivot){
      small++;
      if (small != i)
        swap(arr[small], arr[i]);
    }
  }
  small++;
  swap(arr[small], arr[high]);
  return small;
}

bool checkOverHalf(int* arr, int number, int length){
  int times = 0;
  for (int i = 0; i < length; ++i){
    if (arr[i] == number)
      times++;
  }
  return times * 2 <= length ? false : true;
}

int FindOverhalfNumber(int* arr, int length){
  if (arr == NULL || length <= 0)
    return -1;
  int middle = length >> 1;
  int low = 0, high = length - 1;
  int pos = partition(arr, length, low, high);
  while (pos != middle){
    if (pos < middle){
      low = pos + 1;
      pos = partition(arr, length, low, high);
    }
    else{
      high = pos - 1;
      pos = partition(arr, length, low, high);
    }
  }
  int result = arr[pos];
  if (checkOverHalf(arr, result, length))
    return result;
  else
    return -1;
}

void main(int argc, char* argv[])
{
  int arr[] {2, 2, 2, 6, 7, 9, 0, 2, 2, 2, 2, 2};
  cout << FindOverhalfNumber(arr, 12);
  system("pause");
}