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) {}
* };
*/
TreeNode* f(vector<int> &preorder, vector<int> &inorder,int l1,int r1,int l2,int r2);
class Solution {
public:

TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder) {
return f(preorder,inorder,0,preorder.size()-1,0,inorder.size()-1);

}
};

TreeNode* f(vector<int> &preorder, vector<int> &inorder,int l1,int r1,int l2,int r2) {
if(l2>r2) return NULL;
int i=l2,cnt=0;
for (;inorder[i]!=preorder[l1]&&i<=r2;i++,cnt++);
if (i>r2) return NULL;
TreeNode* head = new TreeNode(preorder[l1]);
head->left = f(preorder,inorder,l1+1,l1+cnt,l2,i-1);
head->right = f(preorder,inorder,l1+cnt+1,r1,i+1,r2);
return head;
}