hdu 5323 Solve this interesting problem（dfs）

题目链接：hdu 5323 Solve this interesting problem

逆向思维，每次向左或向右翻倍，知道左端点为0时，即恰好满足的情况，处理处所有情况去取最小值。

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

using namespace std;
typedef long long ll;
const ll inf = 0x3f3f3f3f;

ll L, R, N;

void dfs(ll l, ll r) {
//printf("%lld %lld\n", l, r);
if (l <= 0 || r >= 2 * R) {
if (l == 0)
N = min(N, r);
return;
}

int k = r - l + 1;

if (k > l)
return;

dfs(l - k, r);
dfs(l - k - 1, r);

dfs(l, r + k);
if (k > 1)
dfs(l, r + k - 1);
}

int main () {
while (scanf("%lld%lld", &L, &R) == 2) {
N = inf;
dfs(L, R);
printf("%lld\n", N == inf ? -1 : N);
}
return 0;
}