Constructing a binary tree in post-order and in-order
topic
This question requires constructing a binary tree (the number of nodes in the tree does not exceed 10) using the post-order sequence and the in-order sequence, and output its pre-order sequence.
Input format
Enter the number of elements in the first line.
Enter the sequence in the second line, separated by spaces.
Enter the in-order sequence in the third line, separated by spaces.
Output format
Output the preorder sequence of this binary tree, separated by spaces, with a space at the end.
Input examples (and their corresponding binary trees)
5
20 40 50 30 10
20 10 40 30 50
## 输出样例
10 20 30 40 50
code
#include <iostream>
#include <vector>
#include <unordered_map>
class TreeNode {
public:
int val;
TreeNode* left;
TreeNode* right;
TreeNode(int x) : val(x), left(nullptr), right(nullptr) {
}
};
TreeNode* buildTree(std::vector<int>& inorder, std::vector<int>& postorder, int inStart, int inEnd, int postStart, int postEnd, std::unordered_map<int, int>& indexMap) {
if (inStart > inEnd || postStart > postEnd) {
return nullptr;
}
int rootVal = postorder[postEnd];
TreeNode* root = new TreeNode(rootVal);
int rootIndex = indexMap[rootVal];
int leftSubtreeSize = rootIndex - inStart;
root->left = buildTree(inorder, postorder, inStart, rootIndex - 1, postStart, postStart + leftSubtreeSize - 1, indexMap);
root->right = buildTree(inorder, postorder, rootIndex + 1, inEnd, postStart + leftSubtreeSize, postEnd - 1, indexMap);
return root;
}
void preorderTraversal(TreeNode* root) {
if (root == nullptr) {
return;
}
std::cout << root->val << " ";
preorderTraversal(root->left);
preorderTraversal(root->right);
}
int main() {
int n;
std::cin >> n;
std::vector<int> postorder(n);
std::vector<int> inorder(n);
for (int i = 0; i < n; ++i) {
std::cin >> postorder[i];
}
for (int i = 0; i < n; ++i) {
std::cin >> inorder[i];
}
std::unordered_map<int, int> indexMap;
for (int i = 0; i < n; ++i) {
indexMap[inorder[i]] = i;
}
TreeNode* root = buildTree(inorder, postorder, 0, n - 1, 0, n - 1, indexMap);
preorderTraversal(root);
std::cout << std::endl;
return 0;
}