TIME_LIMIT_EXCEEDED at Codeforces 558E - c++

Question

I am beginner in the programming field and I have started to solve problems on Codeforces and this is my first problem on it and when I submit this problem solution A Simple Task with this code
#include <iostream>
#include <string>
using namespace std;
void swapchar(char &x, char &y)
{
char temp;
temp = x;
x = y;
y = temp;
}
void main()
{
string s;
long n, q;
long i, j;
bool k;
cin >> n >> q;
cin >> s;
for (int x = 0; x<q; x++)
{
cin >> i >> j >> k;
if (i<1 || j<1 || i>n || j>n)
break;
if (k == 1)
{
for (int u = i - 1; u < j; u++)
{
for (int v = u + 1; v < j; v++)
{
if (s[u] > s[v])
swapchar(s[u], s[v]);
}
}
}
else if (k == 0)
for (int u = i - 1; u < j; u++)
{
for (int v = u + 1; v < j; v++)
{
if (s[u] < s[v])
swapchar(s[u], s[v]);
}
}
}
cout << s << endl;
}
then the codeforces' output is :-
Time limit exceeded on test 6
and when i searched about what it is the test 6 i found :-
Test: #6, time: 5000 ms., memory: 12 KB, exit code: -1, checker exit code: 0, verdict: TIME_LIMIT_EXCEEDED
Input
2256 44182
kanqevxwgecliptqmdsgnflqyohgtukphlbmjxndbtjqujuafxankfghlseytdwdviamqjscacuyrghriuaihxtyersgnyvigenpflwequgbdusnvlgplxjxkqhjbdvkmufpoirqueufblnnrnbhmcnvewzfdonwjgswuneimtykntwgrlfqlvkdblwjzplhffzqpopbjmvrjcxyzgxqhkjbrgdqnipsipexpoozphfrgzboiiiskawtbhegerhvknrzljclhnpokpazhspsmzeiujddlpfireoyjzriickcuwtbimxjbhunedcdgaabztczkzmahnriarzcmnkjrrfqkodxbpocmxjvutpqbmawcsghwxdidhmwbfxuqegpjtqfvaloycogvoxdtjotlknazaeofaxlomeywwlezlndhpjwbgpxgkvubropxffytucvlbhjugzqgglrezoqsrvwkdrbuehbjxtgobugghqrgbgacqi...
I don't know what does that problem mean and how to solve it
Please explain and help me.

Answer 1

You're using a selection sort (with a few superfluous swaps) that's O(n^2) complexity, try using std::sort instead. You don't have too much to change in your code:
#include <iostream>
#include <string>
#include <algorithm>
#include <functional>
using namespace std;
int main()
{
string s;
long n, q;
long i, j;
bool k;
cin >> n >> q;
cin >> s;
for (int x = 0; x<q; x++)
{
cin >> i >> j >> k;
if (i<1 || j<1 || i>n || j>n)
break;
if (k == 1)
{
std::sort(s.begin() + i-1, s.begin() + j);
}
else if (k == 0)
std::sort(s.begin() + i-1, s.begin() + j, std::greater<char>());
}
cout << s << endl;
}
std::sort(s.begin() + i-1, s.begin() + j); sorts from s.begin() + i-1 up to but not includings.begin() + j. The second just sorts in reverse order by using > (std::greater) instead of < for comparison. Note the #include <algorithm> for the std::sort and #include <functional> for std::greater.
Note that I got rid of your swapchar function since we don't need it anymore, I also changed the return type of main to int as it's supposed to be.
The above code manages to handle the test 6 that you had problems with but exceeds time limit for test 9 (I even tried some small other changes there).
To complete tests 9+ you're probably supposed to think a bit more about the problem and possible input, take for example the following sort queries:
20 57 1
89 950 1
57 100 0
57 100 1
1 9500 0
Here we'd do tons of useless sorting and I wouldn't be surprised if the 9th problem were to test if you identified this. First we sort the range [20, 57] and [89, 950] in ascending order, then we sort the range [57, 100] in descending order and resorting the same range in ascending order right afterwards overriding the first one completely. Lastly we sort the range [1, 9500] in descending order, overwriting all the previous sorts which we could have completely left out since they're overwritten anyways.
We could make use of the knowledge that later sorts can and likely will override previous ones. We could first save all the "sort queries" we were given and later start with the last one (since that one would override all previous ones anyways) and go in reverse order only sorting the ranges we haven't sorted yet. That way we can get rid of lots of useless sorting that we'd otherwise do even though we'd later overwrite it and speeding it up a lot.