https://leetcode.com/problems/find-leaves-of-binary-tree/

Leetcode: 366. Find Leaves of Binary Tree

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func findLeaves(root *TreeNode) [][]int {
    leaves := [][]int{}
    
    leaf := func(depth, v int) {
        if len(leaves) - 1 < depth {
            leaves = append(leaves, []int{v})
        } else {
            leaves[depth] = append(leaves[depth], v)
        }
    }
    
    var traverse func(root *TreeNode, depth int) int /*max depth*/
    traverse = func(root *TreeNode, depth int) int {
        if root == nil {
            return depth-1
        }
        if root.Left == nil && root.Right == nil {
            leaf(0, root.Val)
            return depth
        }
        dl := traverse(root.Left, depth+1)
        dr := traverse(root.Right, depth+1)
        // assign max depth to dl
        if dr > dl {
            dl = dr
        }
        // add leaf at depth of the max - current 
        leaf(dl - depth, root.Val)
        return dl
    }
    _ = traverse(root, 0)
    return leaves
}