Leetcode no. 107

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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);    }}

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