N个元素的有序数组A，判断是否有 A[i]=i 存在

/// <summary>

 //下述算法只对严格单调的整数数组有效，复杂度O(N);
    /// 查找数组中的一个数k，满足a[k] == k
    /// 需保证a数组严格单调上升
    /// </summary>
    /// <returns>找到返回k，找不到返回-1</returns>
    public staticint FindNumber(int[]a)
    {
        if (a== null)
            return -1;
        int left =0, right= a.Length;
        while (left+ 1< right)
        {
            int mid = left+ (right- left)/ 2;
            if (a[mid]> mid)
                right = mid;
            else
                left = mid;
        }
        return a[left]== left ? left: -1;
    }