PROBLEM LINK:
Practice
Contest: Division 1
Contest: Division 2
Contest: Division 3
Contest: Division 4
Author: kulyash
Testers: tabr, iceknight1093
Editorialist: iceknight1093
DIFFICULTY:
TBD
PREREQUISITES:
Binary search
PROBLEM:
F(N) denotes the difference between the number of squares and the number of cubes from 1 to N.
Given X, find the smallest value of N such that F(N) \geq X.
EXPLANATION:
Problems of the type "find the smallest N such that \ldots" are often solved with binary search.
Unfortunately, that requires the function to be monotonic, and F(N) here isn’t monotonic.
Analyzing how it changes, we can see that:
In particular, notice that F can only increase at a square number, so the final answer must be a square number.
Further, the fact that cubes are more ‘spread out’ than squares means that F is actually a monotonic function when evaluated on the squares!
That is, we consider the function g(N) = F(N^2) instead, which is increasing.
Proof
It’s enough to prove that there’s at most one cube lying between any two consecutive squares, since this is what makes F increasing on the squares.
So, suppose we have 1\leq x^2 \leq y^3 \leq (y+1)^3. We just need to prove (x+1)^2 \lt (y+1)^3.
This is not hard to do by simple algebraic manipulation.
Expand out (x+1)^2 and (y+1)^3, to obtain (x^2 + 2x + 1) and (y^3 + 3y + 3y^2 + 1).
Comparing their terms, we can see that:
- y^3 \geq x_2
- They both have a 1
- 3y^2 \geq 3x \gt 2x. This follows from the fact that x^2 \leq y^3 means that y^2 \geq x^{4/3} \geq x.
So we have the required inequality.
With this information in hand, the solution is simple: use binary search to find the smallest value of N such that g(N) \geq X; the final answer is then N^2.
This requires us to quickly evaluate g(N) = F(N^2).
Note that the number of squares that are \leq N^2 is simply N, so we just need to count the number of cubes that are \leq N^2.
There are several ways to do this: for example, use std::cbrtl or yet another binary search.
Regarding precision
You may have encountered precision errors when computing cube roots; luckily, one of the samples was specifically designed to catch several common implementations.
In particular, cbrt in C++ or something like N ** (1 / 3) in Python have precision issues.
There are several ways to get around this, some being problem dependent:
- If possible, the absolute best solution is to simply not deal with floating-point integers at all. In this problem, the number of cubes \leq N can be computed using a binary search across the integers, so this is the safest way.
-
cbrtlin C++ is precise enough to get AC in this task. - The cube root can be manually adjusted once you compute it: if c is the value you compute as the cube root of N (after rounding) and (c+1)^3 \leq N, then increase c to c+1 instead.
- After computing the cube root, add a small value to it (like 10^{-10}) before rounding to an integer.
TIME COMPLEXITY
\mathcal{O}(\log^2 X) per testcase.
CODE:
Setter's code (C++)
#include <bits/stdc++.h>
using namespace std;
#define ll long long
int main() {
vector<ll>cubes;
for(ll i=1;i<=1010000;i++)cubes.push_back(i*i*i);
ll T;
cin >> T;
while(T--){
ll x;
cin >> x;
ll l=1;
ll r=2e9;
ll ans;
while(l<=r){
ll mid=(r+l)/2;
ll temp=upper_bound(cubes.begin(),cubes.end(),mid*mid)-cubes.begin();
ll curr=mid-temp;
if(curr>=x){
ans=mid*mid;
r=mid-1;
}
else{
l=mid+1;
}
}
cout << ans << endl;
}
return 0;
}
Tester's code (C++)
#include <bits/stdc++.h>
using namespace std;
#ifdef tabr
#include "library/debug.cpp"
#else
#define debug(...)
#endif
struct input_checker {
string buffer;
int pos;
const string all = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
const string number = "0123456789";
const string upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const string lower = "abcdefghijklmnopqrstuvwxyz";
input_checker() {
pos = 0;
while (true) {
int c = cin.get();
if (c == -1) {
break;
}
buffer.push_back((char) c);
}
}
int nextDelimiter() {
int now = pos;
while (now < (int) buffer.size() && buffer[now] != ' ' && buffer[now] != '\n') {
now++;
}
return now;
}
string readOne() {
assert(pos < (int) buffer.size());
int nxt = nextDelimiter();
string res;
while (pos < nxt) {
res += buffer[pos];
pos++;
}
// cerr << res << endl;
return res;
}
string readString(int minl, int maxl, const string &pattern = "") {
assert(minl <= maxl);
string res = readOne();
assert(minl <= (int) res.size());
assert((int) res.size() <= maxl);
for (int i = 0; i < (int) res.size(); i++) {
assert(pattern.empty() || pattern.find(res[i]) != string::npos);
}
return res;
}
int readInt(int minv, int maxv) {
assert(minv <= maxv);
int res = stoi(readOne());
assert(minv <= res);
assert(res <= maxv);
return res;
}
long long readLong(long long minv, long long maxv) {
assert(minv <= maxv);
long long res = stoll(readOne());
assert(minv <= res);
assert(res <= maxv);
return res;
}
void readSpace() {
assert((int) buffer.size() > pos);
assert(buffer[pos] == ' ');
pos++;
}
void readEoln() {
assert((int) buffer.size() > pos);
assert(buffer[pos] == '\n');
pos++;
}
void readEof() {
assert((int) buffer.size() == pos);
}
};
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
input_checker in;
int tt = in.readInt(1, 1e5);
in.readEoln();
while (tt--) {
int x = in.readInt(1, 1e9);
in.readEoln();
long long low = 0, high = 2e9;
while (high - low > 1) {
long long mid = (high + low) >> 1;
long long cnt = mid;
long long t = cbrt(mid * mid);
while ((t + 1) * (t + 1) * (t + 1) <= mid * mid) {
t++;
}
while (t * t * t > mid * mid) {
t--;
}
cnt -= t;
if (cnt >= x) {
high = mid;
} else {
low = mid;
}
}
cout << high * high << '\n';
}
in.readEof();
return 0;
}
Editorialist's code (Python)
from bisect import bisect_left
cubes = [i**3 for i in range(1, 1010000)]
for _ in range(int(input())):
x = int(input())
lo, hi = 1, 2 * 10**9
def g(n): # f(n*n)
return n - bisect_left(cubes, n*n + 1)
while lo < hi:
mid = (lo + hi)//2
if g(mid) >= x: hi = mid
else: lo = mid+1
print(lo * lo)