<h3><a title="题目描述" href="https://leetcode-cn.com/problems/count-complete-tree-nodes">题目描述</a></h3>  <h4>方法一</h4> <pre class="EnlighterJSRAW" data-enlighter-language="java"> //未用到完全二叉树性质 class Solution { public int countNodes(TreeNode root) { if(root==null){ return 0; } int res = 0; Queue<TreeNode> queue = new LinkedList<TreeNode>(); queue.add(root); while(queue.size()!=0){ TreeNode node = queue.remove(); res+=1; if(node.left!=null){ queue.add(node.left); } if(node.right!=null){ queue.add(node.right); } } return res; } } </pre> <p> </p> <h4>方法二:二分+位运算</h4> <pre class="EnlighterJSRAW" data-enlighter-language="java"> class Solution { public int countNodes(TreeNode root) { if (root == null) { return 0; } int level = 0; TreeNode node = root; while (node.left != null) { level++; node = node.left; } int low = 1 << level, high = (1 << (level + 1)) - 1; //最少2^level个，最多2^(level+1) - 1个 while (low < high) { int mid = (high - low + 1) / 2 + low; if (exists(root, level, mid)) { low = mid; } else { high = mid - 1; } } return low; } public boolean exists(TreeNode root, int level, int k) { int bits = 1 << (level - 1); TreeNode node = root; while (node != null && bits > 0) { if ((bits & k) == 0) { node = node.left; } else { node = node.right; } bits >>= 1; } return node != null; } } </pre> <p> </p>