Sum of Left Leaves

The problem. Given the root of a binary tree, return the sum of all left leaves. A left leaf is a leaf that is the left child of its parent.

Example

Input:  root = [3, 9, 20, null, null, 15, 7]     Output: 24
Input:  root = [1]                              Output: 0   (the root isn't anyone's left child)

Two conditions must both hold: the node is a leaf (no children) and it is a left child. A node can check the first by itself, but not the second — only its parent knows which side it hangs on.