107_BinaryTreeLevelOrderTraversalII107.java

/**
 * Given a binary tree, return the bottom-up level order traversal of its
 * nodes' values. (ie, from left to right, level by level from leaf to root).
 *
 * For example:
 * Given binary tree [3,9,20,null,null,15,7],
 *     3
 *    / \
 *   9  20
 *     /  \
 *    15   7
 * return its bottom-up level order traversal as:
 * [
 *   [15,7],
 *   [9,20],
 *   [3]
 * ]
 */

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BinaryTreeLevelOrderTraversalII107 {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        LinkedList<List<Integer>> res = new LinkedList<>();
        if (root == null) return res;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while (!q.isEmpty()) {
            int size = q.size();
            List<Integer> level = new ArrayList<>();
            for (int i=0; i<size; i++) {
                TreeNode node = q.remove();
                level.add(node.val);
                if (node.left != null) q.add(node.left);
                if (node.right != null) q.add(node.right);
            }
            res.addFirst(level);
        }
        return res;
    }


    public List<List<Integer>> levelOrderBottom2(TreeNode root) {
        LinkedList<List<Integer>> res = new LinkedList<>();
        if (root == null) return res;
        levelOrder(root, 0, res);
        return res;
    }

    private void levelOrder(TreeNode root, int level, LinkedList<List<Integer>> res) {
        if (root == null) return;
        if (res.size() <= level) res.addFirst(new ArrayList<>());
        levelOrder(root.left, level+1, res);
        levelOrder(root.right, level+1, res);
        res.get(res.size()-level-1).add(root.val);
    }

}