dsa · easy
Sum of Left Leaves
A **leaf** has no children. A **left leaf** is a leaf that is the left child of its parent. The root is never a left leaf (it has no parent).
Arguments
root— binary tree as nested [val, left, right] or None
Return the sum of every left-leaf value in root. Each node is [val, left, right] or None. An empty tree sums to 0.
Example
root = [3, [9, None, None], [20, [15, None, None], [7, None, None]]].
9 is a left child and a leaf → count it. 15 is a left child of 20 and a leaf → count it. 7 is a right child, so skip it. Sum 9 + 15 = 24.
[1, None, [2, None, None]] has a right leaf only → 0.
Constraints
Number of nodes in [0, 1000] -1000 <= val <= 1000 Hidden tests include near-max size for this bound; a slower-than-intended solution TLEs.
Examples
Example 1
Input: [3, [9, None, None], [20, [15, None, None], [7, None, None]]] Expected: 24
Example 2
Input: [1, None, [2, None, None]] Expected: 0