#Java 7, 170 169 bytes

Java 7, 170 169 bytes

#Java 7, 170 bytes

import java.util.*;boolean c(int[]a){List l=new ArrayList();l.add(a[0]);for(int i:a){if(l.indexOf(i)<0&l.indexOf(i-1)<0&l.indexOf(i+1)<0)return 0>1;l.add(i);}return 1>0;} 

Try it here.

import java.util.*; class M{ static boolean c(int[] a){ List l = new ArrayList(); l.add(a[0]); for(int i : a){ if(l.indexOf(i) < 0 & l.indexOf(i-1) < 0 & l.indexOf(i+1) < 0){ return 0>1; //false } l.add(i); } return 1>0; //true } public static void main(String[] a){ System.out.println(c(new int[]{ 0 })); System.out.println(c(new int[]{ 0, 1 })); System.out.println(c(new int[]{ 1, 0 })); System.out.println(c(new int[]{ 0, 1, 2 })); System.out.println(c(new int[]{ 0, 2, 1 })); System.out.println(c(new int[]{ 2, 1, 3, 0 })); System.out.println(c(new int[]{ 3, 1, 0, 2 })); System.out.println(c(new int[]{ 1, 2, 0, 3 })); System.out.println(c(new int[]{ 2, 3, 1, 4, 0 })); System.out.println(c(new int[]{ 0, 5, 1, 3, 2, 4 })); System.out.println(c(new int[]{ 6, 5, 4, 7, 3, 8, 9, 2, 1, 0 })); System.out.println(c(new int[]{ 4, 3, 5, 6, 7, 2, 9, 1, 0, 8 })); System.out.println(c(new int[]{ 5, 2, 7, 9, 6, 8, 0, 4, 1, 3 })); System.out.println(c(new int[]{ 20, 13, 7, 0, 14, 16, 10, 24, 21, 1, 8, 23, 17, 18, 11, 2, 6, 22, 4, 5, 9, 12, 3, 15, 19 })); System.out.println(c(new int[]{ 34, 36, 99, 94, 77, 93, 31, 90, 21, 88, 30, 66, 92, 83, 42, 5, 86, 11, 15, 78, 40, 48, 22, 29, 95, 64, 97, 43, 14, 33, 69, 49, 50, 35, 74, 46, 26, 51, 75, 87, 23, 85, 41, 98, 82, 79, 59, 56, 37, 96, 45, 17, 32, 91, 62, 20, 4, 9, 2, 18, 27, 60, 63, 25, 61, 76, 1, 55, 16, 8, 6, 38, 54, 47, 73, 67, 53, 57, 7, 72, 84, 39, 52, 58, 0, 89, 12, 68, 70, 24, 80, 3, 44, 13, 28, 10, 71, 65, 81, 19 })); System.out.println(c(new int[]{ 47, 48, 46, 45, 44, 49, 43, 42, 41, 50, 40, 39, 38, 51, 37, 36, 52, 35, 34, 33, 32, 53, 54, 31, 30, 55, 56, 29, 28, 57, 58, 59, 60, 27, 26, 61, 25, 62, 63, 64, 65, 66, 67, 24, 23, 22, 21, 68, 69, 20, 19, 18, 17, 70, 71, 16, 15, 72, 73, 74, 75, 76, 14, 13, 12, 77, 11, 10, 9, 8, 78, 7, 79, 80, 6, 81, 5, 4, 3, 82, 2, 83, 84, 1, 85, 86, 87, 0, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 })); } } 

#Java 7, 170 169 bytes

import java.util.*;Object c(int[]a){List l=new ArrayList();l.add(a[0]);for(int i:a){if(l.indexOf(i)<0&l.indexOf(i-1)<0&l.indexOf(i+1)<0)return 0>1;l.add(i);}return 1>0;} 

Try it here.

import java.util.*; class M{ static Object c(int[] a){ List l = new ArrayList(); l.add(a[0]); for(int i : a){ if(l.indexOf(i) < 0 & l.indexOf(i-1) < 0 & l.indexOf(i+1) < 0){ return 0>1; //false } l.add(i); } return 1>0; //true } public static void main(String[] a){ System.out.println(c(new int[]{ 0 })); System.out.println(c(new int[]{ 0, 1 })); System.out.println(c(new int[]{ 1, 0 })); System.out.println(c(new int[]{ 0, 1, 2 })); System.out.println(c(new int[]{ 0, 2, 1 })); System.out.println(c(new int[]{ 2, 1, 3, 0 })); System.out.println(c(new int[]{ 3, 1, 0, 2 })); System.out.println(c(new int[]{ 1, 2, 0, 3 })); System.out.println(c(new int[]{ 2, 3, 1, 4, 0 })); System.out.println(c(new int[]{ 0, 5, 1, 3, 2, 4 })); System.out.println(c(new int[]{ 6, 5, 4, 7, 3, 8, 9, 2, 1, 0 })); System.out.println(c(new int[]{ 4, 3, 5, 6, 7, 2, 9, 1, 0, 8 })); System.out.println(c(new int[]{ 5, 2, 7, 9, 6, 8, 0, 4, 1, 3 })); System.out.println(c(new int[]{ 20, 13, 7, 0, 14, 16, 10, 24, 21, 1, 8, 23, 17, 18, 11, 2, 6, 22, 4, 5, 9, 12, 3, 15, 19 })); System.out.println(c(new int[]{ 34, 36, 99, 94, 77, 93, 31, 90, 21, 88, 30, 66, 92, 83, 42, 5, 86, 11, 15, 78, 40, 48, 22, 29, 95, 64, 97, 43, 14, 33, 69, 49, 50, 35, 74, 46, 26, 51, 75, 87, 23, 85, 41, 98, 82, 79, 59, 56, 37, 96, 45, 17, 32, 91, 62, 20, 4, 9, 2, 18, 27, 60, 63, 25, 61, 76, 1, 55, 16, 8, 6, 38, 54, 47, 73, 67, 53, 57, 7, 72, 84, 39, 52, 58, 0, 89, 12, 68, 70, 24, 80, 3, 44, 13, 28, 10, 71, 65, 81, 19 })); System.out.println(c(new int[]{ 47, 48, 46, 45, 44, 49, 43, 42, 41, 50, 40, 39, 38, 51, 37, 36, 52, 35, 34, 33, 32, 53, 54, 31, 30, 55, 56, 29, 28, 57, 58, 59, 60, 27, 26, 61, 25, 62, 63, 64, 65, 66, 67, 24, 23, 22, 21, 68, 69, 20, 19, 18, 17, 70, 71, 16, 15, 72, 73, 74, 75, 76, 14, 13, 12, 77, 11, 10, 9, 8, 78, 7, 79, 80, 6, 81, 5, 4, 3, 82, 2, 83, 84, 1, 85, 86, 87, 0, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 })); } }