404. Sum of Left Leaves
Easy LeetCodeGiven 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)
Python
# 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)
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