Left Leaves

Problem

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.

Examples

sumOfLeftLeaves([1,2,3,4,5])
// returns 4
// Only the node with value 4 is a left leaf.

sumOfLeftLeaves([5,4,null,3,null,2])
// returns 2
// The node with value 2 stands as the sole left leaf.