Binary_Tree_Level_Order_Traversal_II.md

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]
]

code:

/**
 * 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>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        traverse(root, 1, result);
        Collections.reverse(result);
        return result;
    }

    void traverse(TreeNode root, int level, 
                  List<List<Integer>> result) {
        if (root == null) return;

        if (level > result.size())
            result.add(new ArrayList<>());

        result.get(level-1).add(root.val);
        traverse(root.left, level+1, result);
        traverse(root.right, level+1, result);
    }
}