find_nineseries.c

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

int number_of_digits(int num) {
	int count = 0;
	for (count = 0; num != 0; count++, num /= 10);
	return count;
}

int highest_digit(int num, int nod) {
/*
	while (num > 9) {
		num /= 10;
	}
	return num;
*/
	static const int powers_of_10[33] = {
		1000000000, 1000000000,
		100000000, 100000000, 100000000,
		10000000, 10000000, 10000000,
		1000000, 1000000, 1000000, 1000000,
		100000, 100000, 100000,
		10000, 10000, 10000,
		1000, 1000, 1000, 1000,
		100, 100, 100,
		10, 10, 10,
		1, 1, 1, 1, 1
	};
	int leading_zeros = __builtin_clz(num);
	num /= powers_of_10[leading_zeros];
	return num >= 10 ? 1 : num;
}

bool is_nineseries(int num) {
	int nod = number_of_digits(num);
	int p1_nod = number_of_digits(num + 1);
	int hd = highest_digit(num, nod);
	int p1_hd = highest_digit(num+1, p1_nod);
	if ((nod+1 == p1_nod) || (hd+1 == p1_hd) && (nod > 1)) {
		return true;
	}
	return false;
}

int generate_int_by_nod(int hd, int nod) {
	int i = 1;
	for (i = 1; i < nod; i++) {
		hd *= 10;
	}
	return hd;
}

void output_nineseries(int hd, int nod) {
	printf("%d", hd);
	while (--nod > 0) {
		printf("%d", 9);
	}
}

void find_nineseries(int a, int b) {
	int nod_a = number_of_digits(a);
	int nod_b = number_of_digits(b);
	int hd_a = highest_digit(a, nod_a);
	int hd_b = highest_digit(b, nod_b);
	if (is_nineseries(b)) {
		output_nineseries(hd_b, nod_b);
	} else {
		if (nod_a == nod_b) {
			if (hd_a == hd_b) {
				output_nineseries(hd_b, 1);
				int rcsv_a = a - generate_int_by_nod(hd_a, nod_a);
				int rcsv_b = b - generate_int_by_nod(hd_b, nod_b);
				int gap = nod_b - number_of_digits(rcsv_b);
				while (gap-- > 1) {
					output_nineseries(0, 1);
				}
				find_nineseries(rcsv_a, rcsv_b);
			} else {
				output_nineseries(hd_b-1, nod_b);
			}
		} else {
			if (nod_b == 1) {
				output_nineseries(b, 1);
			} else {
				if (hd_b > 1) {
					output_nineseries(hd_b-1, nod_b);
				} else {
					output_nineseries(9, nod_b-1);
				}
			}
		}
	}
}

int main(int argc, char *argv[]) {
	int a, b;
	if (argc == 1) {
		int i = 0, n = 0;
		scanf("%d", &n);
		for (i = 0; i < n; i++) {
			scanf("%d %d", &a, &b);
			find_nineseries(a, b);
			printf("\n");
		}
	} else if (argc == 3) {
		a = atoi(argv[1]);
		b = atoi(argv[2]);
		if (a < 1 || b > 1000000 || a > b) {
			printf("Error: 1 <= a < b <= 1000000\n");
			return -1;
		}
		find_nineseries(a, b);
		printf("\n");
	} else {
		printf("Error: Please input a and b.\n");
		return -1;
	}
	return 0;
}