0404. Sum of Left Leaves

https://leetcode.com/problems/sum-of-left-leaves

Description

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

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