C++ Matching Brackets 2 Solution not working - c++

Question

This problem which is similar to another that I solved here is giving me a wrong answer even though the algorithm works on the sample case.I have initialized all the variables this time and it works on a modified version of my previous algorithm.
#include <iostream>
int main() {
int n;
std::cin >> n;
int arr[n];
for (int i = 0; i <n ;++i) {
std::cin >> arr[i];
}
int four_count = 0, two_count = 0, three_long=0, one_long = 0 , max1_long = 0 ,max3_long = 0,a_depth = 0,max_depth = 0;
for (int i = 0; i < n; ++i) {
if (arr[i] == 3) {
if (arr[i+1] == 1) {
++a_depth;
if (a_depth > max_depth) {
max_depth = a_depth;
}
}
++four_count;
three_long += 2;
}
if (arr[i] == 1) {
if (arr[i+1] == 3) {
++a_depth;
if (a_depth > max_depth) {
max_depth = a_depth;
}
}
++two_count;
one_long += 2 ;
}
if (arr[i] == 2) {
if (arr[i+1] == 4 && i < n-1) {
--a_depth;
}
--two_count;
}
if (arr[i] == 4) {
if (arr[i+1] == 2 && i < n-1){
--a_depth;
}
--four_count;
}
if (four_count == 0 && two_count == 0) {
if (three_long >= one_long) {
if (three_long > max3_long) {
max3_long = three_long+one_long;
}
three_long = 0;
one_long = 0;
}
else {
if (one_long > max1_long) {
max1_long = one_long+three_long;
}
one_long = 0;
three_long = 0;
}
}
}
std::cout << max_depth*2 << " " << max1_long << " " << max3_long;
std::cout << "\n";
return 0;
}
Here is a link to the problem:
https://www.codechef.com/ZCOPRAC/problems/ZCO12003

Answer 1

In the below code:
for (int i = 0; i < n; ++i) {
if (arr[i] == 3) {
if (arr[i+1] == 1) {
when i reaches n-1, arr[i+1] becomes arr[n] resulting in an out-of-bounds memory access which will lead to undefined behaviour.

Answer 2

Let's say n is equal to 5. That means the array arr has the maximum index 4, because the first one is 0.
In your loop
for (int i = 0; i < n; ++i)
{ if (arr[i] == 3) {
if (arr[i+1] == 1) {
at some point i becomes n-1, so i == 4, then you try arr[i+1] meaning arr[5], which is out of bound.
Note that in the comment to P.Ws post, you tried if (arr[i+1] == 1 && i < n-1) to fix this. That won't work because there still is an arr[i+1] being executed. You could fix this by using
if(i < n-1) {
if(arr[i+1]) {
but that would mean an even deeper nesting of your ifs. You should probably rethink your approach to the given problem.
Edit: Are you sure you mean ++i and not i++?