Validate complete binary tree

You are given the root of a binary tree.

A complete binary tree is defined as a binary tree in which:

1. Every level, except possibly the last, is completely filled.
2. All nodes in the last level are as far left as possible.

Write a function that determines whether the given binary tree is complete.

Return true if the tree is complete, and false otherwise.

You may assume the tree nodes are defined as:

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
}

Examples:

1. Input tree:

      1
     / \
    2   3
   / \ /
  4  5 6

Output: true (all levels are filled except the last, and the last level is left-aligned).

2. Input tree:

      1
     / \
    2   3
   / \   \
  4  5    7

Output: false (node 3 is missing a left child but has a right child, so nodes in the last level are not as far left as possible).

Constraints:

• The number of nodes in the tree is in the range [1, 10^4] .
• Node values are arbitrary integers and are not necessarily unique.