library(dplyr)
library(zoo)

x<-c(0,0,1,1,0,1,1,1,0,1,1,0,1,1)
aaa<-data.frame(x)

a <- tbl_df(aaa) %>%
  mutate(id = (1:length(x)))

b <- a %>%
  filter(x == 0) %>%
  mutate(id2 = (1:length(x))) %>%
  mutate(check = ifelse(as.integer(id2 / 2) == id2 / 2, F, T)) %>%
  select(-id2)
  

out <- a %>%
  full_join(b) %>%
  na.locf() %>%
  mutate(ref = ifelse(check, as.character(id), ''))