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/**
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* File: build_tree.swift
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* Created Time: 2023-09-02
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* Author: nuomi1 (nuomi1@qq.com)
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*/
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import utils
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/* 构建二叉树：分治 */
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func dfs(preorder: [Int], inorderMap: [Int: Int], i: Int, l: Int, r: Int) -> TreeNode? {
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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 nil
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}
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// 初始化根节点
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let root = TreeNode(x: preorder[i])
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// 查询 m ，从而划分左右子树
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let m = inorderMap[preorder[i]]!
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// 子问题：构建左子树
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root.left = dfs(preorder: preorder, inorderMap: inorderMap, i: i + 1, l: l, r: m - 1)
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// 子问题：构建右子树
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root.right = dfs(preorder: preorder, inorderMap: inorderMap, i: i + 1 + m - l, l: m + 1, r: 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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func buildTree(preorder: [Int], inorder: [Int]) -> TreeNode? {
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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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let inorderMap = inorder.enumerated().reduce(into: [:]) { $0[$1.element] = $1.offset }
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return dfs(preorder: preorder, inorderMap: inorderMap, i: inorder.startIndex, l: inorder.startIndex, r: inorder.endIndex - 1)
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}
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@main
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enum BuildTree {
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/* Driver Code */
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static func main() {
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let preorder = [3, 9, 2, 1, 7]
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let 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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let root = buildTree(preorder: preorder, inorder: inorder)
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print("构建的二叉树为：")
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PrintUtil.printTree(root: root)
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}
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}
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