https://leetcode.com/problems/number-of-digit-one/

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

Example:

Input: 13
Output: 6 
Explanation: Digit 1 occurred in the following numbers: 1, 10, 11, 12, 13.

代碼：

class Solution {
public:
    int countDigitOne(int n) {
        int res = 0;
        long long a = 1, b = 1;
        while (n) {
            res += (n + 8) / 10 * a + (n % 10 == 1) * b;
            b += n % 10 * a;
            a *= 10;
            n /= 10;
        }
        return res;
    }
};

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#Leetcode# 233. Number of Digit One