在排序数组中，找出给定数字的出现次数

int find(int *a, int n, int x)
{
    int half;

    if (n <= 0) return 0;
    if (n == 1) return a[0] == x ? 1 : 0;
    if (a[0] > x || a[n - 1] < x) return 0;
    if (a[0] == a[n - 1]) return n;

    half = n / 2;
    if (a[half] > x) return find(a, half, x);
    if (a[half] < x) return find(a + half + 1, n - half - 1, x);
    return 1 + find(a, half, x) + find(a + half + 1, n - half - 1, x);
}