cf 1216e1

https://codeforc.es/problemset/problem/1216/E1

 

求1121231234...序列里面第k个数字，k不超过10亿。 

我们只要预处理一个sum数组，然后每次二分一下（其实不二分也可以） 

 

#include <bits/stdc++.h>
using namespace std;
int const inf = 1e9;
int q, sum[100000], pw[10];
int num(int x)
{
    int res = 0;
    while (x)
    {
        x /= 10;
        res++;
    }
    return res;
}
int query(int t, int x)
{
    while (1)
    {
        x--;
        if (x == 0)
            return t % 10;
        t /= 10;
    }
}
int main()
{
    scanf("%d", &q);
    int s = 0, n;
    pw[0] = 1;
    for (int i = 1; i < 10; i++)
        pw[i] = pw[i - 1] * 10;
    for (int i = 1;; i++)
    {
        s += num(i);
        sum[i] = sum[i - 1] + s;
        if (sum[i] >= inf)
        {
            n = i;
            break;
        }
    }
    while (q--)
    {
        int x;
        scanf("%d", &x);
        int l = 1, r = n;
        while (l < r)
        {
            int mid = (l + r) / 2;
            if (sum[mid] >= x)
                r = mid;
            else
                l = mid + 1;
        }
        x -= sum[r - 1];
        for (int i = 1; i < 10; i++)
        {
            int tmp = (pw[i] - pw[i - 1]) * i;
            if (x > tmp)
                x -= tmp;
            else
            {
                int t = pw[i - 1] + (x - 1) / i;
                x = (x - 1) % i + 1;
                printf("%d\n", query(t, i - x + 1));
                break;
            }
        }
    }
    return 0;
}