Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> findLeaves(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        TreeNode prev = new TreeNode(0, root, null);
        while (prev.left != null) {
            List<Integer> t = new ArrayList<>();
            dfs(prev.left, prev, t);
            res.add(t);
        }
        return res;
    }

    private void dfs(TreeNode root, TreeNode prev, List<Integer> t) {
        if (root == null) {
            return;
        }
        if (root.left == null && root.right == null) {
            t.add(root.val);
            if (prev.left == root) {
                prev.left = null;
            } else {
                prev.right = null;
            }
        }
        dfs(root.left, root, t);
        dfs(root.right, root, t);
    }
}