637. Average of Levels in Binary Tree

Description

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

Note: The range of node's value is in the range of 32-bit signed integer.

Constraints

Approach

Examples

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].

Solutions

// Definition for a binary tree node.
public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode() {}
    TreeNode(int val) { 
        this.val = val;
    }
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

/**
 * Time complexity : O(n). The whole tree is traversed once only. 
 *    Here, n refers to the total number of nodes in the given binary tree.
 * Space complexity : O(h). resres and countcount array of size h are used. 
 *    Here, h refers to the height(maximum number of levels) of the given 
 *    binary tree. Further, the depth of the recursive tree could go upto h only.
 */

public class Solution {
    public List < Double > averageOfLevels(TreeNode root) {
        List < Integer > count = new ArrayList < > ();
        List < Double > res = new ArrayList < > ();
        average(root, 0, res, count);
        for (int i = 0; i < res.size(); i++)
            res.set(i, res.get(i) / count.get(i));
        return res;
    }
    public void average(TreeNode t, 
                        int i, 
                        List<Double> sum, 
                        List<Integer> count) {
        if (t == null)
            return;
        if (i < sum.size()) {
            sum.set(i, sum.get(i) + t.val);
            count.set(i, count.get(i) + 1);
        } else {
            sum.add(1.0 * t.val);
            count.add(1);
        }
        average(t.left, i + 1, sum, count);
        average(t.right, i + 1, sum, count);
    }
}

/**
 * Time complexity : O(n). The whole tree is traversed atmost once. 
 *    Here, n refers to the number of nodes in the given binary tree.
 * Space complexity : O(m). The size of queue or temp can grow upto atmost 
 *    the maximum number of nodes at any level in the given binary tree. 
 *    Here, m refers to the max mumber of nodes at any level in the input tree.
 */

class Solution {
    
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> averageList = new ArrayList<>();
        if(root == null) {
            return averageList;
        }
        Queue<TreeNode> queue = new LinkedList();
        queue.add(root);
        
        while(!queue.isEmpty()) {
            int size = queue.size();
            double sum = 0;
            for(int i = 0; i < size; i++) {
                TreeNode curr = queue.remove();
                sum += curr.val;
                if(curr.left != null) {
                    queue.add(curr.left);
                }
                if(curr.right != null) {
                    queue.add(curr.right);
                }
            }
            averageList.add(sum/size);
        }
        
        return averageList;
    }
}

Follow up

Previous630. Course Schedule III Next645. Set Mismatch

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