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52 lines
1.4 KiB
52 lines
1.4 KiB
1 year ago
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/**
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* File: build_tree.cpp
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* Created Time: 2023-07-17
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* Author: Krahets (krahets@163.com)
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*/
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#include "../utils/common.hpp"
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/* 构建二叉树：分治 */
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TreeNode *dfs(vector<int> &preorder, vector<int> &inorder, unordered_map<int, int> &hmap, int i, int l, int r) {
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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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// 初始化根节点
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TreeNode *root = new 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(vector<int> &preorder, vector<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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unordered_map<int, int> hmap;
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for (int i = 0; i < inorder.size(); 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.size() - 1);
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return root;
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}
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/* Driver Code */
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int main() {
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vector<int> preorder = {3, 9, 2, 1, 7};
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vector<int> inorder = {9, 3, 1, 2, 7};
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cout << "前序遍历 = ";
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printVector(preorder);
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cout << "中序遍历 = ";
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printVector(inorder);
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TreeNode *root = buildTree(preorder, inorder);
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cout << "构建的二叉树为：\n";
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printTree(root);
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return 0;
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}
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