#Leetcode# 233. Number of Digit One
https://leetcode.com/problems/number-of-digit-one/
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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.
Example:
Input: 13 Output: 6 Explanation: Digit 1 occurred in the following numbers: 1, 10, 11, 12, 13.
代码：
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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