Sum Of Left Leaves

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.

Class TreeNode:
    val (int)
    left (TreeNode)
    right (TreeNode)

Input Format:

First Parameter - TreeNode root

Output Format:

Return the number.

Example 1 :

Input:
    root = [3 9 20 null null 15 7]
Output:
    24
Explanation:
    There are two left leaves in the tree, with values 9 and 15.

Example 2 :

Input : 
    root = [1]
Output : 
     0

Contraints :

• The number of nodes in the tree is in the range [1, 1000].
• -1000 <= Node.val <= 1000
• Expected Time Complexity : O(N)
• Expected Space Complexity: O(N)