Codeforces Round #389 Technocup 2017 E. Santa Claus and Tangerines（二分+DP）

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <stack>
#include <ctime>
#include <bitset>
#include <cstdlib>
#include <cmath>
#include <set>
#include <list>
#include <deque>
#include <map>
#include <queue>
#define Max(a,b) ((a)>(b)?(a):(b))
#define Min(a,b) ((a)<(b)?(a):(b))
using namespace std;
typedef long long ll;
typedef long double ld;
const double eps = 1e-6;
const double PI = acos(-1);
const int mod = 1000000000 + 7;
const int INF = 0x3f3f3f3f;
// & 0x7FFFFFFF
const int seed = 131;
const ll INF64 = ll(1e18);
const int maxn = 1e6 + 10;
const int maxv = 1e7 + 10;
int n, a[maxn], d[maxv], vis[maxv], kase = 0;
ll k;
int dp(int cur, int mid) {
if(vis[cur] == kase) return d[cur];
vis[cur] = kase;
if(cur&1) {
if(cur/2 < mid) return d[cur] = 1;
else return d[cur] = dp(cur/2, mid) + dp(cur/2+1, mid);
}
else {
if(cur/2 < mid) return d[cur] = 1;
else return d[cur] = 2*dp(cur/2, mid);
}
}
bool ok(int mid) {
ll cnt = 0;
++kase;
for(int i = 1; i <= n; i++) {
if(a[i] < mid) continue;
cnt += dp(a[i], mid);
}
return cnt >= k;
}
int main() {
scanf("%d%I64d", &n, &k);
for(int i = 1; i <= n; i++) {
scanf("%d", &a[i]);
}
sort(a+1, a+n+1);
int l = 1, r = a
, ans = -1;
while(r >= l) {
int mid = (l + r) >> 1;
if(ok(mid)) ans = max(ans, mid), l = mid+1;
else r = mid-1;
}
printf("%d\n", ans);
return 0;
}