Codeforces Beta Round #91 (Div. 2 Only)深度优先

C. Lucky Sum

对这道题我想了很多方法，最直观的方法是打表法，把要用的数直接打表存起来，然后直接查找就好。算法思路是对的，但是提交的时候一直提示不是时间超时就是占用内存太大，最后看到了别人的代码竟然采用了深度优先搜索的办法给解决了。我对深度优先里的递归一直理解的都不怎么好，先整理下来有时间再看吧。代码如下：

//the code is not written by myself
#include <iostream>
#include<algorithm>
#define LL long long
using namespace std;
const int maxn = 20000;
LL d[maxn];
int tot = 1;
void dfs(LL num, int cur)
{
d[tot++] = num;
if (cur > 11) return;
dfs(num * 10 + 4, cur + 1);
dfs(num * 10 + 7, cur + 1);
}
LL sum(LL x)
{
if (x == 0)
return 0;
LL temp = 0;
for (int i = 1; i < tot; i++)
{
if (x >= d[i])
temp += d[i] * (d[i] - d[i - 1]);
else
{
temp += d[i] * (x - d[i - 1]);
break;
}
}
return temp;
}
int main()
{
dfs(4, 0);
dfs(7, 0);
sort(d + 1, d + tot);
LL lt, rt;
cin >> lt >> rt;
cout << sum(rt) - sum(lt - 1) << endl;
return 0;
}