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55 lines
1.5 KiB
55 lines
1.5 KiB
/**
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* File: build_tree.kt
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* Created Time: 2024-01-25
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* Author: curtishd (1023632660@qq.com)
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*/
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package chapter_divide_and_conquer.build_tree
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import utils.TreeNode
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import utils.printTree
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/* 构建二叉树：分治 */
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fun dfs(
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preorder: IntArray,
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inorderMap: Map<Int?, Int?>,
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i: Int,
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l: Int,
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r: Int
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): TreeNode? {
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// 子树区间为空时终止
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if (r - l < 0) return null
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// 初始化根节点
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val root = TreeNode(preorder[i])
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// 查询 m ，从而划分左右子树
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val m = inorderMap[preorder[i]]!!
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// 子问题：构建左子树
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root.left = dfs(preorder, inorderMap, 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, inorderMap, 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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fun buildTree(preorder: IntArray, inorder: IntArray): 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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val inorderMap = HashMap<Int?, Int?>()
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for (i in inorder.indices) {
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inorderMap[inorder[i]] = i
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}
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val root = dfs(preorder, inorderMap, 0, 0, inorder.size - 1)
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return root
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}
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/* Driver Code */
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fun main() {
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val preorder = intArrayOf(3, 9, 2, 1, 7)
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val inorder = intArrayOf(9, 3, 1, 2, 7)
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println("前序遍历 = ${preorder.contentToString()}")
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println("中序遍历 = ${inorder.contentToString()}")
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val root = buildTree(preorder, inorder)
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println("构建的二叉树为：")
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printTree(root)
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} |