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前言

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计算二叉树的节点个数，计算满二叉树的节点个数，计算完全二叉树的节点个数

一、力扣222. 完全二叉树的节点个数

/*** 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) {if(root == null){return 0;}return 1 + countNodes(root.left) + countNodes(root.right);}
}

更高效的解法

/*** 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) {if(root == null){return 0;}TreeNode l = root, r = root;int low = 0, high = 0;while(l != null){l = l.left;low ++;}while(r != null){r = r.right;high ++;}if(low == high){return (int)Math.pow(2, low) - 1;}return 1 + countNodes(root.left) + countNodes(root.right);}
}