Leetcode 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 {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public IList<IList<int>> LevelOrderBottom(TreeNode root) {
         var result = new List<IList<int>>();
        
        if (root == null) return result;
        
        var q = new Queue<TreeNode>();
        q.Enqueue(root);
        var count = q.Count;
        
        while (q.Count > 0)
        {
            var r = new List<int>();
            
            for (int i = 0; i < count; i++)
            {
                var t = q.Dequeue();
                r.Add(t.val);
                if (t.left != null) q.Enqueue(t.left);
                if (t.right != null) q.Enqueue(t.right);
            }
            
            result.Insert(0, r);
            count = q.Count;
        }
        
        return result;
    }
}