seq <- function(n) {
k2<- k1 <- rep(0, n+2)
c1<-w2<- 1
w1<-c2<-k1[1]<-2
while(w2<=n){
while(c1<w1) {
x<-(c1-1)%%2+1
k2[w2]<-x
w2<-w2+1
if(k1[c1]==2) {
k2[w2]<-x
w2<-w2+1}
c1<-c1+1}
while(c2<w2 && w1<=n) {
x<-c2%%2+1
k1[w1]<-x
w1<-w1+1
if(k2[c2]==2) {
k1[w1]<-x
w1<-w1+1}
c2<-c2+1}}
return(cbind(1:n, k2[1:n]))}
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