Problem URL: Find bottom left tree value

We will traverse the tree with BFS and assign the first node of the each level in our result. At the end of the traversal, we will return the result, which will store the left most value of the last level of the tree.

# 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 findBottomLeftValue(self, root: Optional[TreeNode]) -> int:
        res = None
        q = collections.deque([root])

        while q:
            qLen = len(q)
            for i in range(qLen):
                node = q.pop()
                if i == 0:
                    res = node.val
                if node.left: q.appendleft(node.left)
                if node.right: q.appendleft(node.right)

        return res

Time Complexity: O(n)
Space Complexity: O(n)