二分出要求的数的右边界，在从右边界确定左边界

#include <iostream>
using namespace std;

const int N = 1e5+10;
int a[N];

int main()
{
    int n, q, num;
    scanf("%d%d", &n, &q);
    for(int i = 0; i < n; i++) scanf("%d", &a[i]);
    while(q--)
    {
        scanf("%d", &num);
        int l = 0, r = n-1;
        //二分num的右边界
        while(l < r)
        {
            int mid = l+r+1>>1;
            if(a[mid] <= num) l = mid;//若l == mid区间没缩小，由此看出满足条件后会向右边收拢
            else r = mid-1;
        }
        //求左边界
        int start, end;
        if(a[l] == num)
        {
            start = end = l;
            while(a[--l] == num) start = l;
            printf("%d %d\n", start, end);
        }
        else printf("-1 -1\n");
    }
    return 0;
}