Auto comment: topic has been updated by Xahandd (previous revision, new revision, compare).

Auto comment: topic has been updated by Xahandd (previous revision, new revision, compare).

Accessing memory in the correct order can make your code more cache-friendly and thus much faster. If you access many memory cells that are physically close to each other, the whole block can be kept in the cache.

As an example, try the following code:

const int N = 4096;
int A[N][N];

int main() {
    // initialization
    for (int a=0; a<N; ++a) for (int b=0; b<N; ++b) A[a][b] = rand();

    // iteration in the correct order
    int xor1 = 0;
    for (int a=0; a<N; ++a) for (int b=0; b<N; ++b) xor1 ^= A[a][b];
    cout << xor1 << endl;

    // iteration in the wrong order
    int xor2 = 0;
    for (int b=0; b<N; ++b) for (int a=0; a<N; ++a) xor2 ^= A[a][b];
    cout << xor2 << endl;
}

If you iterate in row major order, you are just reading the array from memory "from left to right". If you iterate in column major order, you are jumping around in memory and cache cannot help you.

On my machine, the second iteration takes cca. 40 times longer than the first one.

You used C++14 in the TLE solution, and C++11 in AC solution. There are some weird things in runtimes in C++14 vs C++11, pointed out here.