Average of Levels in Binary Tree

2017-08-25
#java
leetcode

Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.

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Example 1:
Input:
    3
   / \
  9  20
    /  \
   15   7
Output: [3, 14.5, 11]

Explanation:
The average value of nodes on level 0 is 3, on level 1 is 14.5,
and on level 2 is 11. Hence return [3, 14.5, 11].
Note:
The range of node’s value is in the range of 32-bit signed integer.

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> result = new ArrayList<>();
        Queue<TreeNode> q = new LinkedList<>();

        if(root == null) return result;
        q.add(root);
        while(!q.isEmpty()) {
            int n = q.size(); // this is number of nodes in current level
            double sum = 0.0;
            for(int i = 0; i < n; i++) {
                TreeNode node = q.poll();
                sum += node.val;
                if(node.left != null) q.offer(node.left);
                if(node.right != null) q.offer(node.right);
            }
            result.add(sum / n);
        }
        return result;
    }  
}