Lucky Numbers (easy)

最新推荐文章于 2021-02-23 00:10:47 发布

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本文介绍了幸运数字和超级幸运数字的概念，并讲述了如何找到大于或等于给定正整数n的最小超级幸运数。通过DFS打表的方法解决这个问题，数据规模较小，适合此方法。

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链接：http://codeforces.com/problemset/problem/96/B

Petya loves lucky numbers. Everybody knows that positive integers are lucky if their decimal representation doesn't contain digits other than 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.

Lucky number is super lucky if it's decimal representation contains equal amount of digits 4 and 7. For example, numbers 47, 7744, 474477 are super lucky and 4, 744, 467 are not.

One day Petya came across a positive integer n. Help him to find the least super lucky number which is not less than n.

题意：只有4和7的数字为幸运数字，4和7的个数相同时为超级幸运数字，求给出一个n，从0到n有几个超级幸运数字。

分析：现在看来其实可以用数位dp，不过我还没有很好的掌握，当时用的是简单的dfs打表，毕竟数据不是很大

题解：

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <queue>
#include <stack>
#include <vector>
#include <map>
#include <cstring>
#include <functional>
#include <cmath>
using namespace std;

int k=0;
__int64 table[110];
void dfs(int num,int n4,int n7,int nowp,int allp)
{
	if(n4==n7)
	{
		table[k]=num;
		k++;
	}
	if(nowp==allp)
		return;
	num*=10;
	num+=4;
	dfs(num,n4+1,n7,nowp+1,allp);

	num/=10;
	num*=10;
	num+=7;
	dfs(num,n4,n7+1,nowp+1,allp);
}
int main()
{
	//freopen("in.txt","r",stdin);
	int n;
	dfs(4,1,0,1,8);
	dfs(7,0,1,1,8);
	table[k]=4444477777;
	sort(table,table+110);
	while(~scanf("%d",&n))
	{
		int i;
		for(i=0;i<110;i++)
			if(table[i]>=n)
				break;
		printf("%I64d\n",table[i]);
	}
	return 0;
}