Leetcode: Construct Binary Tree from Preorder and Inorder Traversal

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if(preorder.size()==0||preorder.size()!=inorder.size())
return NULL;
map<int,int> mapper;
for(int i=0;i<inorder.size();++i)
mapper[inorder[i]]=i;
return helper(preorder,inorder,mapper,0,0,preorder.size()-1);
}
TreeNode* helper(vector<int>& preorder,vector<int>& inorder,map<int,int>& mapper,int root,int start,int end){
if(start>end)
return NULL;
int r=preorder[root];
int index=mapper[r];
TreeNode* node= new TreeNode(r);
node->left=helper(preorder,inorder,mapper,root+1,start,index-1);
node->right=helper(preorder,inorder,mapper,root+index-start+1,index+1,end);
return node;
}
};