Number of good leaf nodes pairs

We can traverse the tree and find out the distance between each pair of leaf nodes. If the distance is less than or equal to the given distance, we increment the result by 1.

class Solution:
    def countPairs(self, root: TreeNode, distance: int) -> int:
        self.res = 0

        def dfs(node: TreeNode) -> List[int]:
            if not node:
                return []
            if not node.left and not node.right:
                return [1]
            left = dfs(node.left)
            right = dfs(node.right)
            for i in left:
                for j in right:
                    if i + j <= distance:
                        self.res += 1
            return [i + 1 for i in left + right if i < distance]

        dfs(root)
        return self.res

Time complexity: O(n^2) where n is the number of nodes in the tree.
Space complexity: O(h) where h is the height of the tree.