Note
 Very typical recursion problem.
 The key is to check if cur node’s
left subtree
is a leaf node.
Given the root of a binary tree, return the sum of all left leaves.
Example
1 2 3
 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.

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var sumOfLeftLeaves = function(root) { let result = 0; result = recursion(root); return result; function recursion(node) { if (!node) return 0; if (node.left && !node.left.left && !node.left.right) { return node.left.val + recursion(node.left) + recursion(node.right); } return recursion(node.left) + recursion(node.right); } };
