404. Sum of Left Leaves

Easy LeetCode

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

A leaf is a node with no children. A left leaf is a leaf that is the left child of another node.

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

Constraints

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

How to solve the problem

Code

• Recursion (DFS)

# 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:
        result = 0
        def dfs(node):
            nonlocal result
            if node is None:
                return 
            if node.left and not node.left.left and not node.left.right:
                result += node.left.val
            dfs(node.left)
            dfs(node.right)
        dfs(root)
        return result

Complexity

• Time complexity: O(n), n == number of nodes
• Space complexity: O(n)