Linked List 2.2

Implement an algorithm to find the penultimate in a one-way linked listKNodes.

Analysis: two pointers with k different locations are used to traverse the chain table at the same speed. When the fast pointer is null, the slow pointer just points to the last K node of the linked list. Time complexity O (N) and space complexity O (1 ).

 1 #include <iostream> 2 #include <fstream> 3 #include <assert.h> 4  5 using namespace std; 6  7 struct Node { 8     Node(): val(0), next(0) {} 9     Node( int aval ): val(aval), next(0) {}10     int val;11     Node *next;12 };13 14 Node* findLastKth( Node *node, int k );15 16 int main( int argc, char *argv[] ) {17     string data_file = "./2.2.txt";18     ifstream ifile( data_file.c_str(), ios::in );19     if( !ifile.is_open() ) {20         fprintf( stderr, "cannot open file: %s\n", data_file.c_str() );21         return -1;22     }23     int n = 0, k = 0;24     while( ifile >>n >>k ) {25         assert( n >= 0 );26         Node guard, *node = &guard;27         for( int i = 0; i < n; ++i ) {28             node->next = new Node();29             node = node->next;30             ifile >>node->val;31             cout <<node->val <<" ";32         }33         cout <<endl;34         node = findLastKth( guard.next, k );35         if( node ) {36             printf( "last %dth node is: %d\n", k, node->val );37         } else {38             printf( "last %dth node is: null", k );39         }40         node = guard.next;41         while( node ) {42             Node *next = node->next;43             delete node;44             node = next;45         }46         cout <<endl <<endl;47     }48     ifile.close();49     return 0;50 }51 52 Node* findLastKth( Node *node, int k ) {53     Node *slow = node, *fast = node;54     for( int i = 0; i < k; ++i ) {55         if( !fast ) { return 0; }56         fast = fast->next;57     }58     while( fast ) {59         slow = slow->next;60         fast = fast->next;61     }62     return slow;63 }

Test File

0 00 11 011 111 212 01 32 11 32 21 32 31 37 114 54 96 23 45 12 457 214 54 96 23 45 12 457 714 54 96 23 45 12 45

 

Linked List 2.2