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  • leetcode 222: Count Complete Tree Nodes

    Count Complete Tree Nodes

    Total Accepted: 93 Total Submissions: 678

    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, 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.


    [思路]

    用暴力法, recursive求会超时 O(N).   假设从某节点一直向左的高度 = 一直向右的高度, 那么以该节点为root的子树一定是complete binary tree. 而 complete binary tree的节点数,能够用公式算出 2^h - 1. 假设高度不相等, 则递归调用 return countNode(left) + countNode(right) + 1.  复杂度为O(h^2)  

    [CODE]

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    public class Solution {
        public int countNodes(TreeNode root) {
            if(root==null) return 0;
            
            int l = getLeft(root) + 1;
            int r = getRight(root) + 1;
            
            if(l==r) {
                return (2<<(l-1)) - 1;
            } else {
                return countNodes(root.left) + countNodes(root.right) + 1;
            }
        }
        
        private int getLeft(TreeNode root) {
            int count = 0;
            while(root.left!=null) {
                root = root.left;
                ++count;
            }
            return count;
        }
        
        private int getRight(TreeNode root) {
            int count = 0;
            while(root.right!=null) {
                root = root.right;
                ++count;
            }
            return count;
        }
    }


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  • 原文地址:https://www.cnblogs.com/mthoutai/p/6747991.html
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