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:
TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
return helper(preorder,inorder,0,0,inorder.size()-1);
}
TreeNode* helper(vector<int>& preorder, vector<int>& inorder, int pre, int in_l, int in_r)
{
TreeNode* root=NULL;
int i;
for(i=in_l;i<=in_r;i++)
if(inorder[i]==preorder[pre])//find the root of each subtree
{
root=new TreeNode(preorder[pre]);
break;
}
if(root)
{
root->left=helper(preorder,inorder,pre+1,in_l,i-1);
root->right=helper(preorder,inorder,pre+i-in_l+1,i+1,in_r);
}
return root;
}
};