Tree Level Sum
Easy
Given the root node of a binary tree, implement a function that returns the sum of all nodes' values at a specified level.
Example 1
Input
[ 1, null, 2, 3 ]
Output
3
Explanation. The tree is as follows: 1 \ 2 / 3 Level 2 has one node with value 3.
Example 2
Input
[ 3, 9, 20, null, null, 15, 7 ]
Output
42
Explanation. The tree is as follows: 3 / \ 9 20 / \ 15 7 Level 2 has two nodes, with values 15 and 7, and their sum is 22.
Example 3
Input
[ 1 ]
Output
1
Explanation. The tree has only one node at level 1 with the value 1.
Follow-up: How would your function change if the tree was a binary search tree and you were asked to return the sum of all values between two given levels?
Constraints:
The tree will have at least one node and the given level will always be valid and less than the height of the tree.
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