958. Check Completeness of a Binary Tree #

Problem #

Given the root of a binary tree, determine if it is a complete binary tree.

In a  complete binary tree, every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

Example 1:

Input: root = [1,2,3,4,5,6]
Output: true
Explanation: Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.

Example 2:

Input: root = [1,2,3,4,5,null,7]
Output: false
Explanation: The node with value 7 isn't as far left as possible.

Constraints:

• The number of nodes in the tree is in the range [1, 100].
• 1 <= Node.val <= 1000

Problem Summary #

Given a binary tree, determine whether it is a complete binary tree.

Baidu Baike defines a complete binary tree as follows:

If the depth of a binary tree is h, except for the h-th level, the number of nodes on all other levels (1～h-1) reaches the maximum, and all nodes on the h-th level are continuously concentrated on the far left; this is a complete binary tree. (Note: the h-th level may contain 1~ 2h nodes.)

Solution #

• This problem is a variant of level-order traversal.
• Determine whether each node’s left child is empty.
• Similar problems, Problems 102, 107, and 199, are all level-order traversal.

Code #

package leetcode

import (
	"github.com/halfrost/leetcode-go/structures"
)

// TreeNode define
type TreeNode = structures.TreeNode

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */

func isCompleteTree(root *TreeNode) bool {
	queue, found := []*TreeNode{root}, false
	for len(queue) > 0 {
		node := queue[0] //Take out the first node of each level
		queue = queue[1:]
		if node == nil {
			found = true
		} else {
			if found {
				return false // A nil appears between two non-empty nodes in level-order traversal
			}
			//If the left child is nil, then the appended node.Left is nil
			queue = append(queue, node.Left, node.Right)
		}
	}
	return true
}