404. Sum of Left Leaves

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Find the sum of all left leaves in a given binary tree.
Example:
    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
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Intuition

 If leaf node, and left leaf - contribute to sum
We still need to traverse right child as well, they might have left leaf node
Start with root, recurse top down

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Time - O(n)
Space - O(h)
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