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Given an integer n, count the total number of digit 1 appearing in all non-negative integers less than or equal to n.

For example:
Given n = 13,
Return 6, because digit 1 occurred in the following numbers: 1, 10, 11, 12, 13.

 

Solution:

int countDigitOne(int n) 
{
    int times = 0;
    for (long long int k = 1; k <= n; k *= 10)
    {
        long long int a = n / k, b = n % k;  
        times += (a + 8) / 10 * k;  
        if(a % 10 == 1)
            times += (b + 1);  
    }
    
    return times;
}