404 Sum of Left Leaves

We have to return the sum of all left-leaf values in the tree. This means the left leaf of every subtree. So, it could be on a right child node, just that it has to be a leaf, and it has to be a left leaf at that.

In the code below, we detect any left leaves by the condition root.left and not root.left.left and not root.left.right. If this condition meets, we return the root.left.val and call the function on any right subtree that may or may not be present.

If a leaf is not detected, then make recursive calls on left and right subtrees as usual.

Code:

Python3

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
        if not root: return 0
        
        return \
        root.left.val + self.sumOfLeftLeaves(root.right) \
        if root.left and not root.left.left and not root.left.right \
        else self.sumOfLeftLeaves(root.left) + self.sumOfLeftLeaves(root.right)

Big O Analysis

• Runtime

  The runtime complexity here is $O(N)wheren=number\_of\_nodes$ in the tree.

• Memory

  The memory usage would be of the implicit call stack we would use while recursion.

— A

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