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52 lines
1.5 KiB
52 lines
1.5 KiB
/**
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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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/* Build binary tree: Divide and conquer */
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TreeNode *dfs(vector<int> &preorder, unordered_map<int, int> &inorderMap, int i, int l, int r) {
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// Terminate when subtree interval is empty
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if (r - l < 0)
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return NULL;
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// Initialize root node
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TreeNode *root = new TreeNode(preorder[i]);
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// Query m to divide left and right subtrees
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int m = inorderMap[preorder[i]];
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// Subproblem: build left subtree
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root->left = dfs(preorder, inorderMap, i + 1, l, m - 1);
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// Subproblem: build right subtree
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root->right = dfs(preorder, inorderMap, i + 1 + m - l, m + 1, r);
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// Return root node
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return root;
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}
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/* Build binary tree */
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TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder) {
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// Initialize hash table, storing in-order elements to indices mapping
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unordered_map<int, int> inorderMap;
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for (int i = 0; i < inorder.size(); i++) {
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inorderMap[inorder[i]] = i;
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}
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TreeNode *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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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 << "Pre-order traversal = ";
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printVector(preorder);
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cout << "In-order traversal = ";
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printVector(inorder);
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TreeNode *root = buildTree(preorder, inorder);
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cout << "The constructed binary tree is:\n";
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printTree(root);
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return 0;
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}
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