Sum Of Left Leaves In Binary Tree: Recursive Solution

404. Sum of Left Leaves
Difficulty: Easy
Topics: Tree, Depth-First Search, Breadth-First Search, Binary Tree
Given the root of a binary tree, return the sum of all left leaves.
A leaf is a node with no children. A left leaf is a leaf that is the left child of another node.
Example 1:


Input: root = [3,9,20,null,null,15,7]

Output: 24

Explanation: There are two left leaves in the binary tree, with values 9 and 15 respectively.

Example 2:

Input: root = [1]

Output: 0

Constraints:

The number of nodes in the tree is in the range [1, 1000].
-1000 <= Node.val <= 1000

Solution:
We can implemen...