Level Order Sum

Problem

Given the root of a binary tree, return the sum of the nodes at each level. The output should be a list containing the sum of the nodes at each level.

Example 1

Input:

[1, 3, 4, null, 2, 7, null, 8]

Output: [1, 7, 9, 8]

Example 2

Input:

[1, 2, 5, 3, null, null, null, null, 4]

Output: [1, 7, 3, 4]

Explanation

We should recognize that a level-order breadth-first traversal of the binary tree is the most straightforward way to solve this problem.

At each level, we can keep a running sum of the node’s values at that level. Then, whenever we finish processing a level (the for-loop for that level finishes), then we can add the sum of the nodes to the output list.

class Solution:
    def levelSum(self, root: TreeNode) -> List[int]:
        if not root:
            return []

        nodes = []
        queue = deque([root])

        while queue:
            # start of a new level here
            level_size = len(queue)
            sum_ = 0

            # process all nodes in the current level
            for i in range(level_size):
                node = queue.popleft()
                sum_ += node.val
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)

            # we are at the end of the level,
            # add the sum of the nodes to the output list
            nodes.append(sum_)

        return nodes

class Solution {
    public List<Integer> levelSum(TreeNode root) {
        if (root == null) {
            return new ArrayList<>();
        }

        List<Integer> nodes = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            // start of a new level here
            int levelSize = queue.size();
            int sum = 0;

            // process all nodes in the current level
            for (int i = 0; i < levelSize; i++) {
                TreeNode node = queue.poll();
                sum += node.val;
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }

            // we are at the end of the level,
            // add the sum of the nodes to the output list
            nodes.add(sum);
        }

        return nodes;
    }
}

func levelSum(root *TreeNode) []int {
    if root == nil {
        return []int{}
    }

    var nodes []int
    queue := []*TreeNode{root}

    for len(queue) > 0 {
        // start of a new level here
        levelSize := len(queue)
        sum := 0

        // process all nodes in the current level
        for i := 0; i < levelSize; i++ {
            node := queue[0]
            queue = queue[1:]
            sum += node.Val
            if node.Left != nil {
                queue = append(queue, node.Left)
            }
            if node.Right != nil {
                queue = append(queue, node.Right)
            }
        }

        // we are at the end of the level,
        // add the sum of the nodes to the output list
        nodes = append(nodes, sum)
    }

    return nodes
}

class Solution {
    levelSum(root: TreeNode | null): number[] {
        if (!root) {
            return [];
        }

        const nodes: number[] = [];
        const queue: TreeNode[] = [root];

        while (queue.length > 0) {
            // start of a new level here
            const levelSize = queue.length;
            let sum = 0;

            // process all nodes in the current level
            for (let i = 0; i < levelSize; i++) {
                const node = queue.shift()!;
                sum += node.val;
                if (node.left) {
                    queue.push(node.left);
                }
                if (node.right) {
                    queue.push(node.right);
                }
            }

            // we are at the end of the level,
            // add the sum of the nodes to the output list
            nodes.push(sum);
        }

        return nodes;
    }
}