/** * File: build_tree.cs * Created Time: 2023-07-18 * Author: hpstory (hpstory1024@163.com) */ namespace hello_algo.chapter_divide_and_conquer; public class build_tree { /* 构建二叉树:分治 */ TreeNode? DFS(int[] preorder, Dictionary inorderMap, int i, int l, int r) { // 子树区间为空时终止 if (r - l < 0) return null; // 初始化根节点 TreeNode root = new(preorder[i]); // 查询 m ,从而划分左右子树 int m = inorderMap[preorder[i]]; // 子问题:构建左子树 root.left = DFS(preorder, inorderMap, i + 1, l, m - 1); // 子问题:构建右子树 root.right = DFS(preorder, inorderMap, i + 1 + m - l, m + 1, r); // 返回根节点 return root; } /* 构建二叉树 */ TreeNode? BuildTree(int[] preorder, int[] inorder) { // 初始化哈希表,存储 inorder 元素到索引的映射 Dictionary inorderMap = []; for (int i = 0; i < inorder.Length; i++) { inorderMap.TryAdd(inorder[i], i); } TreeNode? root = DFS(preorder, inorderMap, 0, 0, inorder.Length - 1); return root; } [Test] public void Test() { int[] preorder = [3, 9, 2, 1, 7]; int[] inorder = [9, 3, 1, 2, 7]; Console.WriteLine("前序遍历 = " + string.Join(", ", preorder)); Console.WriteLine("中序遍历 = " + string.Join(", ", inorder)); TreeNode? root = BuildTree(preorder, inorder); Console.WriteLine("构建的二叉树为:"); PrintUtil.PrintTree(root); } }