62 lines
1.9 KiB
C
62 lines
1.9 KiB
C
/**
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* File : build_tree.c
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* Created Time: 2023-10-16
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* Author : lucas (superrat6@gmail.com)
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*/
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#include "../utils/common.h"
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// Assume all elements are less than 1000
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#define MAX_SIZE 1000
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/* Build binary tree: Divide and conquer */
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TreeNode *dfs(int *preorder, int *inorderMap, int i, int l, int r, int size) {
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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 = (TreeNode *)malloc(sizeof(TreeNode));
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root->val = preorder[i];
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root->left = NULL;
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root->right = NULL;
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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, size);
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// Subproblem: build right subtree
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root->right = dfs(preorder, inorderMap, i + 1 + m - l, m + 1, r, size);
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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(int *preorder, int preorderSize, int *inorder, int inorderSize) {
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// Initialize hash table, storing in-order elements to indices mapping
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int *inorderMap = (int *)malloc(sizeof(int) * MAX_SIZE);
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for (int i = 0; i < inorderSize; i++) {
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inorderMap[inorder[i]] = i;
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}
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TreeNode *root = dfs(preorder, inorderMap, 0, 0, inorderSize - 1, inorderSize);
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free(inorderMap);
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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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int preorder[] = {3, 9, 2, 1, 7};
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int inorder[] = {9, 3, 1, 2, 7};
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int preorderSize = sizeof(preorder) / sizeof(preorder[0]);
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int inorderSize = sizeof(inorder) / sizeof(inorder[0]);
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printf("Pre-order traversal =");
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printArray(preorder, preorderSize);
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printf("In-order traversal =");
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printArray(inorder, inorderSize);
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TreeNode *root = buildTree(preorder, preorderSize, inorder, inorderSize);
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printf("The constructed binary tree is:\n");
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printTree(root);
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freeMemoryTree(root);
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return 0;
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}
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