# Binary Tree Level Sum

ID: 482; easy; 二叉树的某层节点之和

## Solution 1 (Java)

/**
* Definition of TreeNode:
* public class TreeNode {
*     public int val;
*     public TreeNode left, right;
*     public TreeNode(int val) {
*         this.val = val;
*         this.left = this.right = null;
*     }
* }
*/

public class Solution {
/**
* @param root: the root of the binary tree
* @param level: the depth of the target level
* @return: An integer
*/
public int levelSum(TreeNode root, int level) {
int sum = 0;
if (root == null || level == 0) return sum;
return bfs(root, sum, level);
}

private int bfs(TreeNode root, int sum, int level) {
List<Integer> result = new ArrayList<>();
Queue<TreeNode> q = new ArrayDeque<>();
q.offer(root);
while (level > 1 && !q.isEmpty()) {
int size = q.size();
int levelSum = 0;
for (int i = 0; i < size; i++) {
TreeNode curr = q.poll();
if (curr.left != null) {
levelSum += curr.left.val;
q.offer(curr.left);
}
if (curr.right != null) {
levelSum += curr.right.val;
q.offer(curr.right);
}
}
level--;
}
return result.get(result.size() - 1);
}
}

• BFS

## Solution 2 (Java)

/**
* Definition of TreeNode:
* public class TreeNode {
*     public int val;
*     public TreeNode left, right;
*     public TreeNode(int val) {
*         this.val = val;
*         this.left = this.right = null;
*     }
* }
*/

public class Solution {
/**
* @param root: the root of the binary tree
* @param level: the depth of the target level
* @return: An integer
*/
public int levelSum(TreeNode root, int level) {
int sum = 0;
if (root == null || level == 0) return sum;
return bfs(root, sum, level);
}

private int bfs(TreeNode root, int sum, int level) {
Queue<TreeNode> q = new ArrayDeque<>();
q.offer(root);
while (!q.isEmpty()) {
int size = q.size();
int levelSum = 0;
level--;
for (int i = 0; i < size; i++) {
TreeNode curr = q.poll();
if (level == 0)
levelSum += curr.val;
if (curr.left != null)
q.offer(curr.left);
if (curr.right != null)
q.offer(curr.right);
}
if (level == 0) return levelSum;
}
return sum;
}
}

### Notes

• Optimized BFS

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