50 lines
1.7 KiB
C#
50 lines
1.7 KiB
C#
/**
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* File: build_tree.cs
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* Created Time: 2023-07-18
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* Author: hpstory (hpstory1024@163.com)
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*/
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namespace hello_algo.chapter_divide_and_conquer;
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public class build_tree {
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/* Build binary tree: Divide and conquer */
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TreeNode? DFS(int[] preorder, Dictionary<int, int> inorderMap, int i, int l, int r) {
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// Terminate when subtree interval is empty
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if (r - l < 0)
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return null;
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// Initialize root node
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TreeNode root = new(preorder[i]);
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// Query m to divide left and right subtrees
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int m = inorderMap[preorder[i]];
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// Subproblem: build left subtree
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root.left = DFS(preorder, inorderMap, i + 1, l, m - 1);
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// Subproblem: build right subtree
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root.right = DFS(preorder, inorderMap, i + 1 + m - l, m + 1, r);
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// Return root node
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return root;
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}
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/* Build binary tree */
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TreeNode? BuildTree(int[] preorder, int[] inorder) {
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// Initialize hash table, storing in-order elements to indices mapping
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Dictionary<int, int> inorderMap = [];
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for (int i = 0; i < inorder.Length; i++) {
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inorderMap.TryAdd(inorder[i], i);
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}
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TreeNode? root = DFS(preorder, inorderMap, 0, 0, inorder.Length - 1);
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return root;
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}
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[Test]
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public void Test() {
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int[] preorder = [3, 9, 2, 1, 7];
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int[] inorder = [9, 3, 1, 2, 7];
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Console.WriteLine("Pre-order traversal = " + string.Join(", ", preorder));
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Console.WriteLine("In-order traversal = " + string.Join(", ", inorder));
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TreeNode? root = BuildTree(preorder, inorder);
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Console.WriteLine("The built binary tree is:");
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PrintUtil.PrintTree(root);
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}
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}
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