力扣105 從前序與中序遍歷構造二叉樹 遞歸

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/**
 * 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:
    unordered_map<int,int> mp;
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        for(int i=0;i<inorder.size();i++)
        {
            mp[inorder[i]]=i;
        }
        return mybulidtree(preorder, inorder,0,preorder.size()-1,0,inorder.size()-1);
    }
    TreeNode* mybulidtree( vector<int>& preorder,  vector<int>& inorder,int preorder_left,int preorder_right,int inorder_left,int inorder_right)
    {
        if (preorder_left > preorder_right) {
             return nullptr;
        }
        int preorder_root=preorder_left;
        int inorder_root=mp[preorder[preorder_root]];
        TreeNode* root=new TreeNode(preorder[preorder_root]);
        int left_size=inorder_root-inorder_left;
        root->left=mybulidtree(preorder,inorder,preorder_left+1,preorder_left+left_size,inorder_left,inorder_root-1);
        root->right=mybulidtree(preorder,inorder,preorder_left+left_size+1,preorder_right,inorder_root+1,inorder_right);
        return root;
    }
};