-(void)RandomButtonImage {

NSMutableArray *images = [NSMutableArray arrayWithObjects:@"Character_1_1.png",@"Character_1_2.png",@"Character_2_1.png",@"Character_2_2.png" nil];

NSArray *Buttons = [NSArray arrayWithObjects:AnsButton1,AnsButton2,AnsButton3,AnsButton4, nil];

for (UIButton *btn in Buttons) {

int randomIndex = random() % images.count;

UIImage *img = [UIImage imageNamed:[images objectAtIndex:randomIndex]];

[btn setImage:img forState:UIControlStateNormal];

[images removeObjectAtIndex:randomIndex];//或者[images removeObjectsAtIndexes:[NSIndexSet indexSetWithIndex:randomIndex]];

}

}