Change Usernames£¨ÍØÆËÐòÁУ©

ÌâÄ¿Á´½Ó ÌâÄ¿ÃèÊö£º You run a web service with $N$ users. The $i$-th user with a current handle $S_i$ wants to change it to $T_i$. Here, $S_1,¡­,$ and $S_N$ are