puzzle #1. <algorithm> is just awesome.

gistfile1.cpp

#include <algorithm>
#include <cstdio>
#include <cstring>

#define BUFSIZE (sizeof("aa/bb/cccc"))

using namespace std;

bool leap(int year)
{
	// Recall that a year is a leap year (has 366 days) if the year is divisible by 4,
	// unless it is divisible also by 100
	// but not by 400

	if (year % 4 != 0)
		return false;

	if (year % 100 != 0)
		return true;

	return (year % 400 == 0);
}

bool valid(int year, int month, int day)
{
	// fix year

	if (year < 1000)
		year += 2000;

	// basic checks

	if (month < 1 || month > 12)
		return false;

	if (day < 1 || day > 31)
		return false;

	// month length

	if (month == 2 && day > 29)
		return false;

	if (day == 31 && (month & 1) == (month > 7))
		return false;

	// leap year

	if (month == 2 && day == 29 && !leap(year))
		return false;

	printf("%d-%02d-%02d\n", year, month, day);

	return true;
}

int main()
{
	char buf[BUFSIZE];
	int vec[3];

	// read the thing
	fgets(buf, BUFSIZE, stdin);

	// remove first \n char
	strtok(buf, "\n");

	// scan ints into our array
	sscanf(buf, "%d/%d/%d", vec, vec+1, vec+2);

	// sort it before using next_permutation
	sort(vec, vec+3);

	do {
		if (valid(vec[0], vec[1], vec[2]))
			return 0;
	}
	while (next_permutation(vec, vec+3));

	printf("%s is illegal\n", buf);

	return 0;
}