1256. Nth Digit

描述

Find the nth digit of the infinite integer sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ...

• n is positive and will fit within the range of a 32-bit signed integer (n < 2^31).

您在真實的面試中是否遇到過這個題？  是

樣例

Example 1:

Input:
3

Output:
3

Example 2:

Input:
11

Output:
0

Explanation:
The 11th digit of the sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ... is a 0, which is part of the number 10.

起初我很粗暴的一個一個添加進去，然後直接取出來

class Solution {
public:
    /**
     * @param n: a positive integer
     * @return: the nth digit of the infinite integer sequence
     */
    int findNthDigit(int n) {
        // write your code here
        string mstr;
        for(long long i=1;i<=n;i++) mstr=mstr+to_string(i);
        return mstr[n-1]-'0';
    }
};

很明顯超時了，所以要進行篩選

class Solution {
public:
    /**
     * @param n: a positive integer
     * @return: the nth digit of the infinite integer sequence
     */
    int findNthDigit(int n) {
        // write your code here
        long long len = 1, cnt = 9, start = 1;
        while (n > len * cnt) {
            n -= len * cnt;
            ++len;
            cnt *= 10;
            start *= 10;
        }
        start += (n - 1) / len;
        string t = to_string(start);
        return t[(n - 1) % len] - '0';
    }
};