Construct Binary Tree from Inorder and Postorder Traversal

题目：

Given inorder and postorder traversal of a tree, construct the binary tree.

跟之前的通过先序遍历和中序遍历构建二叉树一样，有个区别是，这次的根结点每次都从后序列表中从后向前扫。代码：

    TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int idx=postorder.size()-1;
        return buildTree(inorder,postorder,0,inorder.size(),&idx);
    }
    TreeNode *buildTree(vector<int> &inorder,vector<int> &postorder,int start,int len,int* idx){
        if(!len || idx<0) return NULL;
        TreeNode* root=(TreeNode*)malloc(sizeof(TreeNode));
        root->val=postorder[*idx];
        root->left=NULL;
        root->right=NULL;
        int i=start;
        while(i-start<len){
            if(inorder[i]==root->val) break;
            i++;
        }
        
        if(i-start+1<len){
            root->right=buildTree(inorder,postorder,i+1,start+len-i-1,&(--*idx));
        }
        if(i>start){
            root->left=buildTree(inorder,postorder,start,i-start,&(--*idx));
        }
        return root;
    }