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57 lines
1.4 KiB
57 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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/* 构建二叉树：分治 */
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TreeNode? dfs(
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List<int> preorder,
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List<int> inorder,
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Map<int, int> hmap,
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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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// 子树区间为空时终止
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if (r - l < 0) {
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return null;
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}
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// 初始化根节点
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TreeNode? root = TreeNode(preorder[i]);
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// 查询 m ，从而划分左右子树
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int m = hmap[preorder[i]]!;
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// 子问题：构建左子树
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root.left = dfs(preorder, inorder, hmap, i + 1, l, m - 1);
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// 子问题：构建右子树
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root.right = dfs(preorder, inorder, hmap, i + 1 + m - l, m + 1, r);
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// 返回根节点
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return root;
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}
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/* 构建二叉树 */
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TreeNode? buildTree(List<int> preorder, List<int> inorder) {
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// 初始化哈希表，存储 inorder 元素到索引的映射
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Map<int, int> hmap = {};
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for (int i = 0; i < inorder.length; i++) {
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hmap[inorder[i]] = i;
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}
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TreeNode? root = dfs(preorder, inorder, hmap, 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("前序遍历 = $preorder");
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print("中序遍历 = $inorder");
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TreeNode? root = buildTree(preorder, inorder);
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print("构建的二叉树为：");
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printTree(root!);
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}
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