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.
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| 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.
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| 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.
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Constraints:
- The number of nodes in the tree is in the range [1, 100].
- 1 <= Node.val <= 1000
Solution BFS#
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| /**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
int depth = 0;
public boolean isCompleteTree(TreeNode root) {
depth(root, 0);
Queue<TreeNode> q = new LinkedList<>();
q.add(root);
int level = 0;
// System.out.println("depth = " + depth);
while (q.size() > 0) {
int size = q.size();
boolean seenNull = false;
for (int i = 0; i < size; i++) {
TreeNode node = q.poll();
if (level != depth && node == null) {
return false;
}
if (level == depth) {
if (node == null) {
seenNull = true;
} else {
if (seenNull) return false;
}
}
if (node == null) continue;
q.add(node.left);
q.add(node.right);
}
if (level == depth) {
return true;
}
level++;
}
return true;
}
void depth(TreeNode node, int level) {
if (node == null) return;
depth = Math.max(depth, level);
depth(node.left, level + 1);
depth(node.right, level + 1);
}
}
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Solution 2#
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| /**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public boolean isCompleteTree(TreeNode root) {
Queue<TreeNode> q = new LinkedList<>();
q.add(root);
while (q.peek() != null) {
TreeNode node = q.poll();
q.add(node.left);
q.add(node.right);
}
while (q.size() > 0 && q.peek() == null) {
q.poll();
}
return q.size() == 0;
}
}
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