Given the root of a binary tree, return the values of its leaves from left to right (the order in which they would appear if you traced the bottom edge of the drawing).
Examples
1
/ \
2 3
/ \ \
4 5 6
Input: root = build_tree([1, 2, 3, 4, 5, None, 6])
Output: [4, 5, 6]
3
/ \
9 20
/ \
15 7
Input: root = build_tree([3, 9, 20, None, None, 15, 7])
Output: [9, 15, 7]
Constraints
0 <= number of nodes <= 2000-10**4 <= node.val <= 10**4- Target complexity: O(n) time.
Goals
- Collect leaves in left-to-right order
- Choose a traversal order that visits leaves left to right
- Return an empty list for an empty tree