958. Check Completeness of a Binary Tree
1. Description
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.
2. Example
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.
3. Constraints
- The number of nodes in the tree is in the range [1, 100].
- 1 <= Node.val <= 1000
4. Solutions
Breadth-First Search
n is the number of nodes in root
Time complexity: O(n)
Space complexity: O(n)
class Solution {
public:
bool isCompleteTree(TreeNode *root) {
traverse_(root);
return complete_;
}
private:
bool complete_ = true;
void traverse_(TreeNode *root) {
queue<TreeNode *> nodes({root});
bool have_left_child = true;
bool have_right_child = true;
while (!nodes.empty()) {
auto node = nodes.front();
nodes.pop();
if (node->left != nullptr) {
if (!have_right_child) {
complete_ = false;
}
have_left_child = true;
nodes.push(node->left);
} else {
have_left_child = false;
}
if (node->right != nullptr) {
if (!have_left_child) {
complete_ = false;
}
have_right_child = true;
nodes.push(node->right);
} else {
have_right_child = false;
}
}
}
};