958. Check Completeness of a Binary Tree

958. Check Completeness of a Binary Tree (Medium)

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 2^h 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.

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Related Topics:
Tree

Solution 1.

Use level order traversal. After visiting a node that has at least one null child, all the subsequent nodes must be leaf nodes.

// OJ: https://leetcode.com/problems/check-completeness-of-a-binary-tree/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(N)
class Solution {
public:
    bool isCompleteTree(TreeNode* root) {
        if (!root) return true;
        queue<TreeNode*> q;
        q.push(root);
        bool onlyLeafNow = false;
        while (q.size()) {
            int cnt = q.size();
            while (cnt--) {
                root = q.front();
                q.pop();
                if (!root->left && root->right) return false;
                if (onlyLeafNow && (root->left || root->right)) return false;
                if (root->left) q.push(root->left);
                if (root->right) q.push(root->right);
                if (!root->left || !root->right) onlyLeafNow = true;
            }
        }
        return true;
    }
};