[AcWing 789] 数的范围

---

点击查看代码

#include<iostream>

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

int main()
{
    int n, k;
    scanf("%d %d", &n, &k);
    for (int i = 0; i < n; i++) scanf("%d", &a[i]);
    while (k--) {
        int x;
        scanf("%d", &x);
        int l = 0, r = n - 1;
        while (l < r) {
            int mid = l + r >> 1;
            if (a[mid] >= x)    r = mid;
            else    l = mid + 1;
        }
        if (a[l] != x)  printf("-1 -1\n");
        else {
            printf("%d ", l);
            int l = 0, r = n - 1;
            while (l < r) {
                int mid = l + r + 1 >> 1;
                if (a[mid] <= x)    l = mid;
                else    r = mid - 1;
            }
            printf("%d\n", l);
        }
    }
    return 0;
}

---

1. 用到了两个二分模板，当更新公式为 l = mid 时，需要 mid = l + r + 1 >> 1；