ProBlem14

#include <iostream>
using namespace std;
int main()
{
	int i, j, maxl = 0, res;
	for( i = 1000000; i >= 1; i --)
	{
		int c = 0;
		__int64 j = i;
		if(i % 10000 == 0)
			cout<<i<<endl;
		while(j != 1)
		{
			if(j % 2 == 0)
				j/=2;
			else j = j * 3 + 1;
			c ++;
		}
		if(c > maxl)
		{
			maxl = c;
			res = i;
		}
	}
	cout<<res<<endl;
}

answer 837799

Problem15

#include <iostream>
using namespace std;
#define MAX 21
__int64 dp[MAX][MAX];
int main()
{
	memset(dp, 0, sizeof(dp));
	dp[0][0] = 1;
	for(int i = 0; i <= 20; i ++)
	{
		for(int j = 0; j <= 20; j ++)
		{
			if(i-1 >= 0)
				dp[i][j] += dp[i-1][j];
			if(j-1 >= 0)
				dp[i][j] += dp[i][j-1];
		}
	}
	printf("%I64d\n", dp[20][20]);
}

answer 137846528820

Problem16

#include <iostream>
using namespace std;
char str[1000];
char tmp[1000];
int main()
{
	for(int i = 0; i < 999; i ++)
		str[i] = '0';
	str[i] = '1';
	str[i+1]= '\0';
	for( i = 1; i <= 1000; i ++)
	{
		int jw = 0, a;
		for(int j = 999; j >= 0 ; j --)
		{
			a = (str[j] - '0') * 2;
			tmp[j] = (a + jw ) % 10 + '0';
			jw = (a  + jw)/10;
		}
		tmp[1000] = '\0';
		for(j = 0; j < 1000; j ++)
			str[j] = tmp[j];
	
	}
	cout<<str<<endl;
	int sum = 0;
	for(i = 0; i < 1000; i ++)
		sum += str[i] - '0';
	cout<<sum<<endl;
}

answer 1366