求解特定数字的个数问题

求解特定数字的个数问题

求解从1到n的所有数中,特定数字m(0 <= m <= 9)的个数问题

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uint64_t countN(uint64_t x, uint64_t n)
{
    uint64_t lowernum;
    uint64_t currentnum;
    uint64_t highernum;
    uint64_t factor = 1;

    uint64_t ret = 0;

    while (0 < x / factor) {
        highernum = x / (10 * factor);
        currentnum = x / factor % 10;
        lowernum = n - (n / factor * factor);

        if (currentnum < n) {
            ret += highernum * factor;
        } else if (currentnum == n) {
            ret += highernum * factor + lowernum + 1;
        } else {
            ret += (highernum + 1) * factor;	
        }

        factor *= 10;
    }

    return ret;
}

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