 # ALIEN1 - Editorial

Practice

Contest

Author: Prajit Benerjee

Tester: Md Shahid

Editorialist: Md Shahid

Easy

Binary search

# PROBLEM:

There are 10^{18} alien building numbered 1 through 10^{18} i.e, A_i = i. Power of the ith building is P_i,

P_i = \lceil{\frac{2A_i+M}{K}}\rceil , where M and K are positive integer constants.
You are given power X of human, it means that human can break those buildings whose power is less than equal to X and human can only win if they break more than 10^9 alien buildings.

# QUICK EXPLANATION:

You need to first find the maximum number of building human can break using binary search. Take low = 1 and high = 10^{18} in binary search. if number of maximum building human can break is greater than 10^9 then print “YES” otherwise print “NO” with maximum number of building human have broken.

# EXPLANATION:

It is a divide and conquer problem. As you can see number of building is numbered 1 to 10^{18}, so you can not find maximum number of building by iterating. If you carefully observe that building are of sorted order numbered from 1 to 10^{18}. As we already know binary search uses sorted order array but here we don’t need to use array of length 10^{18}, instead we can just use mid = \frac{low+high}{2}.Then check if power at point mid is less and equal X if it is less and equal X, then check for larger number by updating low=mid+1 because we want to find the maxium A_i . If power is greater than X, it means the maximum number lies below mid, then check for lower number by updating high=mid-1. Repeat this process untill high \geq low. When the loop exit, you get your maximum number of building human can break.

### Algorithm :

power(mid, M, K, X) {
p = ceil((2 * mid + M) / 2);
if p <= X
return true;
else
return false;
}

binarySearch(low, high, M, K, X) {
ans = 0
while(high >= low) {
mid = (low + high) / 2;
if power(mid, M, K, X) == true
low = mid + 1
ans = mid
else
high = mid - 1
}
if ans > 1000000000
print "YES"
else
print "NO"
print ans
}


# SOLUTIONS:

Author's Solution
#include<bits/stdc++.h>
#define int long long int
#define ld long double
#define fast ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
using namespace std;
bool f(ld point,ld m,ld k,int x){
int ans;
ans=ceil((2*point+m)/k);
if(ans<=x) return true;
else return false;
}

signed main(){
fast
int t;
cin>>t;
while(t--) {
int n,i,x;
ld m,k;
cin>>m>>k>>x;
int lo=1,hi=(int)1e18,mid,ans=0;
while(hi>=lo){
mid=(hi+lo)/2;
if(f(mid,m,k,x)==true){
lo=mid+1;
ans=mid;
}
else{
hi=mid-1;
}
}
if(ans>(int)1e9){
cout<<"YES"<<endl;
}
else{
cout<<"NO"<<endl;
}
cout<<ans<<endl;
}
return 0;
}

Test's and Editorialist's Solution
def power(mid, M, K, X):
p = (2*mid+M)//K
if (2*mid + M)%K!=0:
p += 1
if p <=X:
return True
else:
return False

def binary_search(low, high, M, K, X):
ans = 0
while(high >= low):
mid = (low+high)//2
if power(mid, M, K, X):
low = mid + 1
ans = mid
else:
high = mid - 1
if ans > pow(10, 9):
print("YES")
else:
print("NO")
print(ans)

for _ in range(int(input())):
M, K, X = map(int, input().split())
low, high = 1, pow(10, 18)
binary_search(low, high,M, K, X)


If you have better approach, please do not hesitate to share with us, we will love your approach. All the suggestions are welcome. 3 Likes