题目描述
blablabla
样例
blablabla
算法1
(暴力枚举) $O(n^2)$
blablabla
时间复杂度
参考文献
C++ 代码
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
map<int, int> hash;
vector<int> preorder, inorder;
TreeNode* buildTree(vector<int>& _preorder, vector<int>& _inorder) {
preorder = _preorder, inorder = _inorder;
for ( int i = 0; i < inorder.size(); i ++ ) hash[inorder[i]] = i;
return dfs(0, preorder.size() - 1, 0, inorder.size() - 1);
}
TreeNode* dfs(int pl, int pr, int il, int ir)
{
if (pl > pr) return nullptr;
auto root = new TreeNode(preorder[pl]);
int k = hash[root->val];
root->left = dfs(pl + 1, pl + k - il, il, k - 1);
root->right = dfs(pl + k - il + 1, pr, k + 1, ir);
return root;
}
};