＃3 Digit Counts

题目描述：

Count the number of k's between 0 and n. k can be 0 - 9.

Have you met this question in a real interview? 

Yes

Example

if n = 12, k = 1 in

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

we have FIVE 1's (1, 10, 11, 12)

题目思路：

这题。。我就是死做。。遍历0～n，每个数字都算一下k出现的次数，然后加起来。。

Mycode（AC ＝ 130ms）：

class Solution {
public:
    /*
     * param k : As description.
     * param n : As description.
     * return: How many k's between 0 and n.
     */
    int digitCounts(int k, int n) {
        // write your code here
        int count = 0;
        for (int i = 0; i <= n; i++) {
            count += countHelper(to_string(i), k);
        }
        return count;
    }
    
    int countHelper(string num, int k) {
        int count = 0;
        for (int i = 0; i < num.size(); i++) {
            if (num.substr(i, 1) == to_string(k)) {
                count++;
            }
        }
        return count;
    }
};