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§6 Leftist Heaps CHAPTER 5 Graph Algorithms Heap: Structure Property + Order Property Target : Speed up merging in O(N). Leftist Heap: Order Property – the same Structure Property – binary tree, but unbalanced 1/10 Discussion 11: How fast can we merge two heaps if we simply use the original heap structure?

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§6 Leftist Heaps 【 Definition 】 The null path length, Npl(X), of any node X is the length of the shortest path from X to a node without two children. Define Npl(NULL) = –1. Note: Npl(X) = min { Npl(C) + 1 for all C as children of X } Note: Npl(X) = min { Npl(C) + 1 for all C as children of X } 【 Definition 】 The leftist heap property is that for every node X in the heap, the null path length of the left child is at least as large as that of the right child. 0 00 10 1 00 10 10 1 The tree is biased to get deep toward the left. 2/10

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§6 Leftist Heaps 【 Theorem 】 A leftist tree with r nodes on the right path must have at least 2 r – 1 nodes. Proof: By induction on p. 162. Trouble makers: Insert and Merge Note: Insertion is merely a special case of merging. 3/10 Discussion 12: How long is the right path of a leftist tree of N nodes? What does this conclusion mean to us?

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§6 Leftist Heaps struct TreeNode { ElementType Element; PriorityQueue Left; PriorityQueue Right; int Npl; } ; Declaration: Merge (recursive version): 3 10 2114 23 8 17 26 6 12 1824 33 7 3718 H1H1 H2H2 Step 1 : Merge( H 1 ->Right, H 2 ) 8 17 26 6 12 1824 33 7 18 37 Step 2 : Attach( H 2, H 1 ->Right ) 3 10 2114 238 17 26 6 12 1824 33 7 18 37 Step 3 : Swap(H 1 ->Right, H 1 ->Left ) if necessary 4/10

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§6 Leftist Heaps PriorityQueue Merge ( PriorityQueue H1, PriorityQueue H2 ) { if ( H1 == NULL ) return H2; if ( H2 == NULL ) return H1; if ( H1->Element Element ) return Merge1( H1, H2 ); else return Merge1( H2, H1 ); } static PriorityQueue Merge1( PriorityQueue H1, PriorityQueue H2 ) { if ( H1->Left == NULL ) /* single node */ H1->Left = H2;/* H1->Right is already NULL and H1->Npl is already 0 */ else { H1->Right = Merge( H1->Right, H2 ); /* Step 1 & 2 */ if ( H1->Left->Npl Right->Npl ) SwapChildren( H1 );/* Step 3 */ H1->Npl = H1->Right->Npl + 1; } /* end else */ return H1; } What if Npl is NOT updated? T p = O(log N) 5/10

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§6 Leftist Heaps Merge (iterative version): 3 10 2114 23 8 17 26 6 12 1824 33 7 3718 H1H1 H2H2 Step 1 : Sort the right paths without changing their left children 3 10 2114 23 6 12 1824 33 7 37 188 17 26 Step 2 : Swap children if necessary 3 10 2114 23 6 12 1824 33 7 37 18 8 17 26 DeleteMin: Step 1 : Delete the root Step 2 : Merge the two subtrees T p = O(log N) Home work: p.181 5.16 Merge two given leftist heaps p.183 5.22 BuildHeap in O(N) 6/10

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§7 Skew Heaps -- a simple version of the leftist heaps Target : Any M consecutive operations take at most O(M log N) time. Merge: Always swap the left and right children except that the largest of all the nodes on the right paths does not have its children swapped. No Npl. Not really a special case, but a natural stop in the recursions. 3 10 2114 23 8 17 26 6 12 1824 33 7 3718 H1H1 H2H2 3 10 2114 23 6 12 1824 33 7 37 26 8 17 18 This is NOT always the case. 7/10

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3 10 2114 23 6 12 1824 3318 7 37 26 8 17 §7 Skew Heaps 【 Example 】 Insert 15 15 10 21 3 6 12 1824 3318 7 37 26 8 17 14 23 15 Merge (iterative version): 3 10 2114 23 8 17 26 6 12 1824 33 7 3718 H1H1 H2H2 3 10 2114 23 6 12 1824 33 7 37 26 8 17 18 8/10

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§7 Skew Heaps Note: Skew heaps have the advantage that no extra space is required to maintain path lengths and no tests are required to determine when to swap children. It is an open problem to determine precisely the expected right path length of both leftist and skew heaps. Note: Skew heaps have the advantage that no extra space is required to maintain path lengths and no tests are required to determine when to swap children. It is an open problem to determine precisely the expected right path length of both leftist and skew heaps. Home work: p.183 5.24 Insertions 9/10

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Research Project 6 Heap Merging (25) This project requires you to implement Merge operation on ordinary heaps, leftist heaps, and skew heaps. You are to analyze and compare the performances of a sequence of merges on these heap structures. Detailed requirements can be downloaded from http://acm.zju.edu.cn/dsaa/ 10/10

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1 Chapter 6 Priority Queues (Heaps) General ideas of priority queues (Insert & DeleteMin) Efficient implementation of priority queue Uses of priority queues.

1 Chapter 6 Priority Queues (Heaps) General ideas of priority queues (Insert & DeleteMin) Efficient implementation of priority queue Uses of priority queues.

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