README.md

233. 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 1:

Input: n = 13
Output: 6

Example 2:

Input: n = 0
Output: 0

Constraints:

• 0 <= n <= 109

Solutions (Rust)

1. Solution

impl Solution {
    pub fn count_digit_one(n: i32) -> i32 {
        let digits = n
            .to_string()
            .bytes()
            .map(|x| (x - b'0') as i32)
            .collect::<Vec<_>>();
        let mut tenpow_r = 10_i32.pow(digits.len() as u32 - 1);
        let mut part_l = 0;
        let mut part_r = n;
        let mut ret = 0;

        for &digit in &digits {
            part_r = n % tenpow_r;
            ret += part_l * tenpow_r
                + match digit {
                    0 => 0,
                    1 => part_r + 1,
                    _ => tenpow_r,
                };
            tenpow_r /= 10;
            part_l = part_l * 10 + digit;
        }

        ret
    }
}