Leetcode no. 107

107. Binary Tree Level Order Traversal II

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 Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res= new LinkedList<>();
        if (root== null) return res;
        level(res, 0, root);
        return res;
    }
    private void level(List<List<Integer>> list, int level, TreeNode root){
        if (root== null) return;
        if (level>= list.size()) list.add(0, new LinkedList<>());
        level(list, level+1, root.left);
        level(list, level+1, root.right);
        list.get(list.size()-level-1).add(root.val);
    }
}