struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

/**
 * Build tree base on element of postorder in [i, j), and element iof inorder [m, n)
 * j-i == n-m
 */
TreeNode *buildTree(vector<int> &postorder, int i, int j, vector<int> &inorder, int m, int n) {
    if (j <= i) return NULL;
    
    int val = postorder[j-1];
    TreeNode *node = new TreeNode(val);

    int index = m;
    while(index < n && inorder[index] != val) ++index;

    if (index == n) {
        // this should be error if happen
        return NULL;
    }

    node->left = buildTree(postorder, i, index-m+i, inorder, m, index);
    node->right = buildTree(postorder, index-m+i, j-1, inorder, index+1, n);

    return node;
}

TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder) {
    // parameters position of inorder and postorder are swapped
    buildTree(postorder, 0, postorder.size(), inorder, 0, inorder.size());
}