The problem statement can be found here.

Description (short):


The input stream contains a set of integer numbers \$A_i\$ (\$0 \le A_i \le 10^{18}\$). The numbers are separated by any number of spaces and line breaks. A size of the input stream does not exceed 256 KB.


For each number \$A_i\$ from the last one until the first one, you should output its square root. Each square root should be printed in a separate line with at least four digits after the decimal point.

#include <iostream>
#include <iomanip>
#include <math.h>
using namespace std;

int main()
    double t;
    double v [128 * 1024];
    int idx = 0;
    while (cin >> t) {
        v[idx] = sqrt(t);

    cout << fixed;
    cout << setprecision(4);

    for (int i = idx - 1; i >= 0; --i)
        cout << v[i] << endl;

    return 0;

I am encountering a TLE on test case 9, with an execution time of 2.031s and memory usage of 1 474 KB. (The allowable limits are 2.0s and 64 MB.) How can I improve (or change) my method to complete the problem successfully?


2 Answers 2


The input stream contains a set of integer numbers \$A_i\$ (\$0 \leq A_i \leq 10^{18}\$).

So you should change

double t;


unsigned long long t;

This actually speeds up the program a lot.

To speed up the output, instead of

cout << fixed;
cout << setprecision(4);

for (int i = idx - 1; i >= 0; --i)
    cout << v[i] << endl;


for (int i = idx - 1; i >= 0; --i) {
    printf("%.4f\n", v[i]);
  • \$\begingroup\$ Why does changing double to unsigned long long speed up the program (after I made these modifications I got AC)? Thanks for the answer! \$\endgroup\$
    – manan
    Commented Jan 22, 2015 at 5:37
  • \$\begingroup\$ well the question was marked C++ so recommending printf seems not right. instead maybe one should turn on buffering of output of cout if printing speed is a problem. \$\endgroup\$
    – AndersK
    Commented Jan 22, 2015 at 5:39
  • 1
    \$\begingroup\$ @manan I would guess it's because for double it will correctly parse things like 1e5, and so is more complicated. I don't really know C++ though, perhaps someone else can chime in. \$\endgroup\$
    – mjolka
    Commented Jan 22, 2015 at 5:41

Some cycles might be shaved off by making v global, and/or replacing index increment/decrement with a pointer increment/decrement. For a smart compiler with necessary optimization turned on it would make no difference; do you know how do they compile the submissions?

As for the review, the code is OK except

  • using namespace std, which is not OK.

  • Possible buffer overflow when number of integers exceeds \$2^{18}\$ (I know they promised, but for code review it is a red flag)

  • Misbehaving on a malformed input (see above...)

  • \$\begingroup\$ Thanks! Is using namespace std generally not ok, or is it acceptable for contest programming? I read the linked answer and it seemed to be discussing larger programs where serious maintenance would be necessary. \$\endgroup\$
    – manan
    Commented Jan 22, 2015 at 5:40
  • \$\begingroup\$ It is a bad habit. The sooner you drop it, the better. \$\endgroup\$
    – vnp
    Commented Jan 22, 2015 at 6:02

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