alternate(a)
n.someone who takes the place of another person 同义词：surrogatereplacement
v. go back and forth; swing back and forth between two states or conditions 同义词：jump
exchange people temporarily to fulfill certain jobs and functions be an understudy or alternate for a role 同义词：understudy
reverse (a direction, attitude, or course of action) 同义词：interchangetackswitchflipflip-flop
do something in turns 同义词：take turns
adj. every second one of a series"the cleaning lady comes on alternate Wednesdays"
allowing a choice 同义词：alternative
occurring by turns; first one and then the other 同义词：alternating(a)
of leaves and branches etc; first on one side and then on the other in two ranks along an axis; not paired"stems with alternate leaves"