106. Construct Binary Tree from Inorder and Postorder 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* build_tree(vector<int>& postorder, int l1,int r1,vector<int>& inorder,int l2,int r2,unordered_map<int,int> &m)
{
if(l1>r1) return NULL;
TreeNode* root=new TreeNode(postorder[r1]);
if(l1==r1) return root;
int i=m[postorder[r1]];
root->left=build_tree(postorder,l1,l1+i-l2-1,inorder,l2,i-1,m);
root->right=build_tree(postorder,l1+i-l2,r1-1,inorder,i+1,r2,m);
return root;
}
TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
unordered_map<int,int> m;
for(int i=0;i<inorder.size();i++) m[inorder[i]]=i;
return build_tree(postorder,0,inorder.size()-1,inorder,0,inorder.size()-1,m);
}
};