数字 1 的个数

/*
 * Solution Idea using Java Streams:
 * We can utilize Java Streams to streamline the process of counting '1's. By creating a range
 * from 0 to n, we can use map and filter operations to count the occurrences of '1'. This
 * approach provides a functional and concise way to implement the solution.
 */
 
import java.util.stream.IntStream;
 
public class Solution {
    public int countDigitOne(int n) {
        return IntStream.rangeClosed(0, n)
                        .map(Solution::countOnesInNumber)
                        .sum();
    }
 
    // Helper method to count the number of '1's in a single number
    private static int countOnesInNumber(int num) {
        int count = 0;
        while (num > 0) {
            if (num % 10 == 1) {
                count++;
            }
            num /= 10;
        }
        return count;
    }
}