LeetCode 404. Sum of Left Leaves



Given the root of a binary tree, return the sum of all left leaves.

Example 1:

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.

Example 2:

Input: root = [1]
Output: 0


  • The number of nodes in the tree is in the range [1, 1000].
  • -1000 <= Node.val <= 1000


Traverse and only sum the left leaves.

Python Solution

# 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: TreeNode) -> int:
        self.sum_left = 0
        self.traverse(root, False)
        return self.sum_left
    def traverse(self, root, is_left):
        if not root:
        if is_left and not root.left and not root.right:
            self.sum_left += root.val            
        self.traverse(root.left, True)
        self.traverse(root.right, False)
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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