Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 2 | Linked Lists
1 LinkedListNode FindBeginning(LinkedListNode head) {
2 LinkedListNode n1 = head;
3 LinkedListNode n2 = head;
4
5 // Find meeting point
6 while (n2.next != null) {
7 n1 = n1.next;
8 n2 = n2.next.next;
9 if (n1 == n2) {
10 break;
11 }
12 }
13
14 // Error check - there is no meeting point, and therefore no loop
15 if (n2.next == null) {
16 return null;
17 }
18
19 /* Move n1 to Head. Keep n2 at Meeting Point. Each are k steps
20 /* from the Loop Start. If they move at the same pace, they must
21 * meet at Loop Start. */
22 n1 = head;
23 while (n1 != n2) {
24 n1 = n1.next;
25 n2 = n2.next;
26 }
27 // Now n2 points to the start of the loop.
28 return n2;
29 }
n1 and n2 will meet here, 3
nodes from start of loop
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