mode

library(plyr)
mat = t(replicate(3,sample(as.character(1:3),100,replace=T)))
DF = data.frame(mat, stringsAsFactors = FALSE)
DF = colwise(factor, levels = as.character(1:3))(DT)
mode_f = function(v){
  tv <- tabulate(v)
  if (sum(tv == max(tv)) > 1){
    NA
  } else {
    levels(v)[which.max(tv)]
  }
}
new_sample = as.character(colwise(mode_f)(DF))
na_loc = which(new_sample == "NA")
for (j in na_loc){
  distance = rowSums(sweep(as.matrix(DF[,1:(j-1)]), 2, new_sample[1:(j-1)], '=='))
  if (sum(distance == min(distance)) > 1){
    new_sample[j] = DF[sample(which(distance == min(distance)), 1),j]
  } else {
    new_sample[j] = DF[which.min(distance),j]
  }
}