Today's leetcode challenge was a binary tree question: sum all of the left leaves of a binary tree.

```
Find the sum of all left leaves in a given binary tree.
Example:
3
/ \
9 20
/ \
15 7
There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
```

My method was using dfs and recursion. I created a dfs function that had two parameters: a value for the node (root) and a boolean value for whether the node was left or not(left). Then I checked to see if the root had a left child or right child and if it was left. If true, I added the value of the node to a cache. I then used recursion to loop through all the left nodes and all the right nodes. In my main function, I set cache = [0] and called the dfs function with parameters of root, and False.

```
class Solution:
def sumOfLeftLeaves(self, root: TreeNode) -> int:
def dfs(root, left):
if not root:
return
if left and not root.left and not root.right:
cache[0] += root.val
dfs(root.left, True)
dfs(root.right, False)
cache = [0]
dfs(root, False)
return cache[0]
```

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