leetcode - 105. Construct Binary Tree from Preorder and Inorder Traversal

/**
 * 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:
    void bt(TreeNode*  & root,vector<int>& preorder,int p1,int p2,vector<int>& inorder,int i1,int i2,unordered_map<int,int>& map)
    {
        if(p1==p2) return ;
        int val=preorder[p1];
        root =new TreeNode(val);
        int i=map[val];
        bt(root->left,preorder,p1+1,i-i1+p1+1,inorder,i1,i,map);
        bt(root->right,preorder,i-i1+p1+1,p2,inorder,i+1,i2,map);
        return ;
    }
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        TreeNode* root(NULL);
        int n=preorder.size();
        unordered_map<int,int>map;
        for(int i=0;i<n;++i)
        {
            map[inorder[i]]=i;
        }
        bt(root,preorder,0,n,inorder,0,n,map);
        return root;
    }
};