【LeetCode】Construct Binary Tree from Inorder and Postorder Traversal

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

Note:
You may assume that duplicates do not exist in the tree.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
   struct TreeNode {
         int val;
         TreeNode *left;
         TreeNode *right;
         TreeNode(int x) : val(x), left(NULL), right(NULL) {}
     };

TreeNode *build(vector &inorder, int st_i,int end_i , vector &postorder, int st_p, int end_p)
{
    if (st_i == end_i)//this tree contains only one node
    {
        TreeNode *root = new TreeNode(inorder[st_i]);
        return root;
    }
    TreeNode *root = new TreeNode(postorder[end_p]);
    int left_len = 0;
    for (int i = st_i; i <= end_i; i++)
    {
        if (inorder[i] == postorder[end_p])//find the root node
        {
            left_len = i-st_i;
            break;
        }
    }
    if (left_len > 0)//left subtree exist
    {
        root->left = build(inorder, st_i, left_len+st_i-1, postorder, st_p, st_p+left_len-1);
    }
    if (st_i+left_len < end_i)//right subtree exist
    {
        root->right = build(inorder, st_i+left_len+1, end_i, postorder, st_p+left_len, end_p-1);
    }
    return root;
}

TreeNode *buildTree(vector &inorder, vector &postorder)
{
    if (inorder.size() == 0 || postorder.size() == 0)
    {
        return NULL;
    }

    TreeNode *root = new TreeNode(postorder.back());
    int left_child_length = 0;
    for (int i = 0; i < inorder.size(); i++)
    {
        if (postorder.back() == inorder[i])
        {
            left_child_length = i;
            break;
        }
    }
    if (left_child_length > 0)
    {
        root->left = build(inorder, 0, left_child_length-1, postorder, 0, left_child_length -1);
    }
    if (left_child_length < inorder.size()-1)
    {
        root->right = build(inorder, left_child_length + 1, inorder.size()-1, postorder, left_child_length, postorder.size()-2);
    }
    return root;

}


};