leetcode(958)：判断是否为完全二叉树

题目

Medium

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

Definition of a complete binary tree from Wikipedia:
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: [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: [1,2,3,4,5,null,7]
Output: false
Explanation: The node with value 7 isn’t as far left as possible.

思路

完全二叉树
除最后一层，其他层都满了
如果最后一层没满，那么没满的全在右边
满二叉树是特殊完全二叉树

利用队列按层遍历，某节点出现空子节点后，之后的节点不可以有子节点，设置一个flag用于判定条件

class Solution {
public:
	bool isCompleteTree(TreeNode* root) {

		if (root == nullptr)
			return true;
		queue<TreeNode*> queueTree;
		queueTree.push(root);
		bool flag = true;
		while (!queueTree.empty()) {

			TreeNode* top = queueTree.front();
			if (top->left != nullptr&&flag)
				queueTree.push(top->left);
			else if (!flag&&top->left != nullptr)
				return false;
			else if (flag&&top->left == nullptr)
			{
				flag = false;
			}

			if (top->right != nullptr&&flag)
				queueTree.push(top->right);
			else if (!flag&&top->right != nullptr)
				return false;
			else if (flag&&top->right == nullptr)
			{
				flag = false;
			}
			queueTree.pop();


		}

		return true;
	}
};