【leetcode】400. 第 N 位数字

首先需要确定这个数字是谁
最重要的一点是 将第几个转换成index
就是index = n-1
这个很重，对于后面的取余运算

using ll = long long;

class Solution {
public:
    
    int findNthDigit(int n) {
        
        ll base = 0;
        ll s = 0;
        while(1){
            ll t = pow(10,base+1)-pow(10,base);
            if(s + t*(base+1)<n){
                s += t*(base+1);
                ++base;
            }else{
                break;
            }
        }
        
        int beg = pow(10,base);
        --n;
        int wei = base+1;
        int num = beg + (n-s)/wei;

        int d = ((n-s)%wei);
        
        char a[12];
        snprintf(a,20,"%d",num);
        return a[d]-'0';
    }
};