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38 | /**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
TreeNode* buildTreeImpl(int preStart, int inStart, int inEnd,
vector<int>& preorder,
vector<int>& inorder,
std::unordered_map<int, int>& inOrderMap) {
if (inStart > inEnd)
return nullptr;
int nodeVal = preorder[preStart];
TreeNode* node = new TreeNode(nodeVal);
int inMidIdx = inOrderMap[nodeVal];
int leftSize = inMidIdx - inStart;
node->left = buildTreeImpl(preStart + 1, inStart, inMidIdx - 1,
preorder, inorder, inOrderMap);
node->right = buildTreeImpl(preStart + 1 + leftSize, inMidIdx + 1, inEnd,
preorder, inorder, inOrderMap);
return node;
}
TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
std::unordered_map<int, int> inOrderMap;
for (int i = 0; i < inorder.size(); i++)
inOrderMap[inorder[i]] = i;
return buildTreeImpl(0, 0, inorder.size() - 1, preorder, inorder, inOrderMap);
}
};
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