[剑指offer] 37. 数字在排序数组中出现的次数

题目描述

统计一个数字在排序数组中出现的次数。

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思路：

已知是排序数组，利用二分查找到k的上下界，相减

class Solution
{
  public:
    int GetNumberOfK(vector<int> data, int k)
    {
        if (data.empty())
            return 0;
        int low = getLowerK(data, 0, data.size() - 1, k);
        int high = getHigherK(data, 0, data.size() - 1, k);
        cout << low << " " << high << endl;
        if (low > -1 && high > -1)
            return (high - low + 1);
        return 0;
    }
    int getLowerK(vector<int> &data, int beg, int end, int k)
    {
        if (beg > end)
            return -1;
        int mid = beg + (end - beg) / 2;
        if (data[mid] == k)
        {
            if ((mid == beg) || (data[mid - 1] != k))
                return mid;
            else
                end = mid - 1;
        }
        else if (data[mid] > k)
            end = mid - 1;
        else
            beg = mid + 1;
        return getLowerK(data, beg, end, k);
    }

    int getHigherK(vector<int> &data, int beg, int end, int k)
    {
        if (beg > end)
            return -1;
        int mid = beg + (end - beg) / 2;
        if (data[mid] == k)
        {
            if ((mid == end) || (data[mid + 1] != k))
                return mid;
            else
                beg = mid + 1;
        }
        else if (data[mid] > k)
            end = mid - 1;
        else
            beg = mid + 1;
        return getHigherK(data, beg, end, k);
    }
};