day53（1.4）——leetcode面试经典150

222. 完全二叉树的节点个数

我感觉会有更简单的方法，但是我不知道，我这几题二叉树，只有一题是用bfs，所以这题我也用了bfs

222. 完全二叉树的节点个数

题目：

题解：

/**
 * 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 int countNodes(TreeNode root) {
        int sum=0;
        if(root == null) {
            return 0;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        sum++;
        while(queue.size() > 0) {
            //poll是取出并移除，peek是查看
            TreeNode treeNode = queue.poll();
            if(treeNode.left != null) {
                queue.offer(treeNode.left);
                sum++;
            }
            if(treeNode.right != null) {
                queue.offer(treeNode.right);
                sum++;
            }
        }
        return sum;
    }
}