Problem

You are given the root of a binary tree. A leaf is a node with no children. A left leaf is a leaf node that is the left child of its parent.

Return the sum of all left leaves in the tree.

Notes

• The root itself is not considered a left leaf.
• If the tree has no left leaves, return 0.

Intuition

This is a tree traversal problem where you need to keep track of whether the current node was reached as a left child.