1+ 'use strict' ;
2+ class Node {
3+ constructor ( data ) {
4+ this . data = data ;
5+ this . next = this . previous = null ;
6+ }
7+ }
8+ 9+ function findPairsSumOfX ( list , x ) {
10+ let count = 0 ;
11+ let head = list ,
12+ tail = list . previous ;
13+ while ( head != tail ) {
14+ let sum = head . data + tail . data ;
15+ if ( sum == x ) {
16+ head = head . next ;
17+ tail = tail . previous ;
18+ count ++ ;
19+ continue ;
20+ } else if ( sum > x ) tail = tail . previous ;
21+ else if ( sum < x ) head = head . next ;
22+ }
23+ return count ;
24+ }
25+ 26+ function displayList ( list ) {
27+ let output = ''
28+ let temp = list ;
29+ while ( temp . next != list ) {
30+ output = output + temp . data + ' ' ;
31+ temp = temp . next ;
32+ }
33+ output = output + temp . data + ' ' ;
34+ console . log ( output ) ;
35+ }
36+ 37+ let list = new Node ( 1 ) ;
38+ list . next = new Node ( 2 ) ;
39+ list . next . previous = list ;
40+ 41+ list . next . next = new Node ( 3 ) ;
42+ list . next . next . previous = list . next ;
43+ 44+ list . next . next . next = new Node ( 4 ) ;
45+ list . next . next . next . previous = list . next . next ;
46+ 47+ list . next . next . next . next = new Node ( 5 ) ;
48+ list . next . next . next . next . previous = list . next . next . next ;
49+ 50+ list . next . next . next . next . next = new Node ( 6 ) ;
51+ list . next . next . next . next . next . previous = list . next . next . next . next ;
52+ 53+ list . previous = list . next . next . next . next . next ;
54+ list . next . next . next . next . next . next = list ;
55+ displayList ( list ) ;
56+ 57+ let x = 5 ;
58+ console . log ( 'Pairs of ' + x + ' = ' + findPairsSumOfX ( list , x ) ) ;
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