acwing 二分

• 如：1，3，3，3，5，6，8，9
• 找左端点的3：
  • mid = (right - left) / 2 + left;
  • if(nums[mid] < key)
    left = mid + 1;
    else
    right = mid;
• 找右端点的3：
  • mid = (right - left) / 2 + left + 1;
  • if(nums[mid] > key)
    right = mid - 1;
    else
    left = mid;

#include <iostream>
#include <vector>

using namespace std;

int find_left(const vector<int>& nums, int left, int right, int key)
{
    while(left < right)
    {
        int mid = (right - left) / 2 + left;
        if(nums[mid] < key)
        {
            left = mid + 1;
        }
        else
        {
            right = mid;
        }
    }
    return nums[left] == key ? left : -1;
}

int find_right(const vector<int>& nums, int left, int right, int key)
{
    
    while(left < right)
    {
        int mid = (right - left) / 2 + left + 1;
        if(nums[mid] > key)
        {
            right = mid - 1;
        }
        else
        {
            left = mid;
        }
    }
    return nums[left] == key ? left : -1;
}
int main()
{
    int n, k;
    cin >> n >> k;
    vector<int> nums(n), qry(k);
    for(int i = 0; i < n; i++)
        cin >> nums[i];
    for(int i = 0; i < k; i++)
        cin >> qry[i];
    //cout << find_left(nums, 0, n - 1, qry[2]);
    //cout << find_right(nums, 0, n - 1, qry[1]);
    for(auto &a : qry)
    {
        cout << find_left(nums, 0, n - 1, a) << " ";
        cout << find_right(nums, 0, n - 1, a) << endl;
    }
    return 0;
}