Can someone help me?

I just solved problem 439C - Devu and Partitioning of the Array. But I was stuck a very long time. First I used the erase method to delete vector element. But I got TLE. The code's here: 6865570. After a long time I changed to use back and pop_back method. The code's here: 6865483. And I got AC! I don't know why erase method takes my time so much. Do you guys have any ideas?