change continuos pop to a simple swap for efficiency

[chunyang-wen]

• change continuous pop to a simple swap for efficiency
• remove redundant empty spaces or tabs

[chunyang-wen]

replace continuous pop

while (!q.empty()) q.pop();

with

std::queue<std::string>().swap(q);

remove redundant spaces or tabs before or after each line.

[Hanjun-Dai]

Thanks!