poj1019

说是组合数学但是根本不是嘛╮(╯▽╰)╭
讲连续的1234.。。看成一组那么我们可以大概得出m组（m*m/2 < 2147483647)，那开到100000稳稳的
然后减掉最后一组之前的那些组的sum_of_lenth值问题就转变成了一个形如123456789101112.。。的第n个字符.

#include<stdio.h>
#include<algorithm>
#include<iostream>
using namespace std;
const long long int maxa = 100005;
long long int a[maxa], sum[maxa];
int main(){
    //freopen("in.cpp", "r", stdin);
    long long int l = 1;
    long long int m = 1;
    a[1] = 1;
    sum[1] = 1;
    for(long long int i = 2; i < maxa; i++){
        if(m*10 == i){
            m = m*10;
            l++;
        }
        a[i] = a[i-1]+l;
        sum[i] = sum[i-1]+a[i];
    }
    long long int n, t;
    cin>>t;
    while(t--){
        cin>>n;
        long long int m;
        for(long long int i = 0; i < maxa; i++){
            if(sum[i] < n){
                m = i;
            }else{
                n -= sum[m];
                break;
            }
        }//printf("*%d
", n);
        long long int l = 1;
        m = 1;
        for(long long int i = 1; i < maxa; i++){
            if(m*10 == i){
                m *= 10;
                l ++;
            }//printf("++%d
", n);
            if(n - l <= 0){m = i;break;}
            else n -= l;
        }
        long long int u = l -n+1;//printf("*%d %d
", m, u);
        while(u){
            u--;
            if(!u){
                printf("%d
", m%10);
            }
            m/=10;
        }
    }
}