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56 lines
1.4 KiB
56 lines
1.4 KiB
/**
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* File: build_tree.dart
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* Created Time: 2023-08-10
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* Author: liuyuxin (gvenusleo@gmail.com)
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*/
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import '../utils/print_util.dart';
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import '../utils/tree_node.dart';
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/* 構建二元樹：分治 */
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TreeNode? dfs(
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List<int> preorder,
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Map<int, int> inorderMap,
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int i,
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int l,
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int r,
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) {
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// 子樹區間為空時終止
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if (r - l < 0) {
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return null;
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}
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// 初始化根節點
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TreeNode? root = TreeNode(preorder[i]);
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// 查詢 m ，從而劃分左右子樹
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int m = inorderMap[preorder[i]]!;
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// 子問題：構建左子樹
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root.left = dfs(preorder, inorderMap, i + 1, l, m - 1);
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// 子問題：構建右子樹
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root.right = dfs(preorder, inorderMap, i + 1 + m - l, m + 1, r);
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// 返回根節點
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return root;
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}
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/* 構建二元樹 */
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TreeNode? buildTree(List<int> preorder, List<int> inorder) {
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// 初始化雜湊表，儲存 inorder 元素到索引的對映
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Map<int, int> inorderMap = {};
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for (int i = 0; i < inorder.length; i++) {
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inorderMap[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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/* Driver Code */
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void main() {
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List<int> preorder = [3, 9, 2, 1, 7];
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List<int> inorder = [9, 3, 1, 2, 7];
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print("前序走訪 = $preorder");
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print("中序走訪 = $inorder");
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TreeNode? root = buildTree(preorder, inorder);
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print("構建的二元樹為：");
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printTree(root!);
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}
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