Leetcode OJ 222. Count Complete Tree Nodes

Total accepted:32628 Total submissions:129569 difficulty:medium

Given a complete binary tree, count the number of nodes.

Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, was completely filled, and all nodes As far left as possible. It can has between 1 and 2h nodes inclusive at the last level H.

If the total count of nodes is returned with brute force, then the time-out will be exceeded, and the complexity is O (N). Since we are asking for the total number of nodes in a binary tree, it is not appropriate to use the common binary tree method to solve the problem. Some of the features of a complete binary tree may be our idea of solving this problem: if the depth of the left subtree of a node is the same as the depth of the right subtree, then the number of nodes in the tree with that node as the root node is: h2-1. In the best case, the time complexity is O (h), and the time complexity of other cases is O (H2). The code is as follows:

1 /**2 * Definition for a binary tree node.3 * public class TreeNode {4 * int val;5 * TreeNode left;6 * TreeNode right;7 * TreeNode (int x) {val = x;}8  * }9  */Ten  Public classSolution { One      Public intcountnodes (TreeNode root) { A         if(root==NULL)return0;  -            -         intL = getLeft (root) + 1;  the         intR = getRight (root) + 1;  -            -         if(l==r) { -             return(2<< (L-1))-1;  +}Else {   -             returnCountnodes (Root.left) + countnodes (root.right) + 1;  +         }   A     }   at        -     Private intgetLeft (TreeNode root) { -         intCount = 0;  -          while(root.left!=NULL) {   -Root =Root.left;  -++count;  in         }   -         returncount;  to     }   +        -     Private intgetRight (TreeNode root) { the         intCount = 0;  *          while(root.right!=NULL) {   $Root =Root.right; Panax Notoginseng++count;  -         }   the         returncount;  +     }   A}

Leetcode OJ 222. Count Complete Tree Nodes