12. Trees 2

# 12. Trees 2 - Maggie Johnson Handout#12 CS103B Structural...

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Maggie Johnson Handout #12 CS103B Key Topics * Structural Induction * Other Methods of Proof for Trees * Node Induction _____________________________________________________________________ In 103A, we saw a number of inductive proofs concerning integers and sequences. We would assume some statement for n, or for all integers less than or equal to n (strong induction), and use this "inductive hypothesis" to prove the same statement for n+1. A similar form of proof is useful for concluding facts about trees. Suppose we want to prove that a statement S(T) is true for all trees. For a basis, we show that S(T) is true whenever T consists of one node. For the induction, we suppose that T is a tree with root r and children c1, c2, . .., ck. Let T1, T2, . .., Tk be the subtrees of T rooted at c1, c2, . .., ck respectively as shown below: The inductive step is to assume S(T1), S(T2), . .., S(Tk) are all true, and prove S(T). If we can do this, we have proven S(T). Why does structural induction work? Example 1: S(T) denotes that there are at most m h leaves in an m-ary tree of height h. (Note: the root is at level 1 with a height of 0) basis: Consider an m-ary tree of height 1. These trees consist of a root with no more than m children, each of which is a leaf. Therefore, there are no more than m 1 = m leaves in an m- ary tree of height 1. inductive hypothesis : We assume the above statement is true for all m-ary trees of height less than h, and show it is true for all m-ary trees of height h. PROOF: Let T be an m-ary tree of height h. The leaves of T are the leaves of the subtrees of T obtained by deleting the edges from the root to each of the vertices connected to it, as shown below:

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Each of these subtrees has height less than or equal to h-1. So, by the inductive hypothesis, these trees have at most m (h-1) leaves. Since there are at most m such subtrees, each with a minimum of m (h-1) leaves, there are at most m * m (h-1) = m h leaves in the rooted tree. Thus, by the principle of mathematical induction applied to the structure of trees, S(T) is true for all m-ary trees. Example 2: S(T) denotes a strictly binary tree with n leaves has 2n-2 edges. (Note: strictly binary means every node has either 0 or 2 children.) basis: A strictly binary tree with two leaves consists of the root connected to its only two children: 2 * 2 - 2 = 2. inductive hypothesis : We assume S(T) is true for all strictly binary trees with i leaves where i >= 2 and i < n leaves; Show that S(T) is true for a tree with n leaves. PROOF: Consider a tree T with n leaves where n > 1. By the definition of a binary tree, T must consist of two subtrees T L and T R (since n > 1). If T L has n L leaves and T R has n R leaves, then n L + n R = n. By the inductive hypothesis, T L has 2n L - 2 edges and T R has 2n R - 2 edges. Since T has two additional edges that are not part of T L or T R , the number of edges in T is (2n L - 2) + (2n R - 2) + 2 = 2(n L + n R ) - 2 = 2n - 2. Thus, by the principle of mathematical induction applied to the structure of trees, S(T) is true
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12. Trees 2 - Maggie Johnson Handout#12 CS103B Structural...

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