I was solving problem https://mirror.codeforces.com/problemset/problem/601/A in which I was using simple BFS but I took MLE on test 57, but when I changed the way of marking visited node slightly it got accepted. Can anyone help me to understand the reason for this ?

This approach was Accepted. https://mirror.codeforces.com/contest/601/submission/55706140

This approach gave MLE https://mirror.codeforces.com/problemset/submission/601/55703452

The only difference is in the way how I mark visited node.

Thank You !!