404. Sum of Left Leaves

Easy

Given the root of a binary tree, return the sum of all left leaves.

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