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  • 958. Check Completeness of a Binary Tree

    Given a binary tree, determine if it is a complete binary tree.

    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.

    Example 1:

    Input: [1,2,3,4,5,6]
    Output: true
    Explanation: Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.
    

    Example 2:

    Input: [1,2,3,4,5,null,7]
    Output: false
    Explanation: The node with value 7 isn't as far left as possible.
    
     

    Note:

    1. The tree will have between 1 and 100 nodes.

    BFS, level order traversal

    用一个boolean flag表示气泡,一旦遇到气泡就可以不用再看其他节点

    time: O(n), space: O(n)

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    class Solution {
        public boolean isCompleteTree(TreeNode root) {
            if(root == null) {
                return true;
            }
            Queue<TreeNode> q = new LinkedList<>();
            q.offer(root);
            boolean flag = false;
            while(!q.isEmpty()) {
                TreeNode cur = q.poll();
                if(cur.left == null) {
                    flag = true;
                } else if(flag) {
                    return false;
                } else {
                    q.offer(cur.left);
                }
                
                if(cur.right == null) {
                    flag = true;
                } else if(flag) {
                    return false;
                } else {
                    q.offer(cur.right);
                }
            }
            return true;
        }
    }
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  • 原文地址:https://www.cnblogs.com/fatttcat/p/10278440.html
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