tsanmmmm2000's Tech Blog

Question

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]
]

Thinking

階序走訪(Level Order Traversal)基本上就是利用廣度優先搜尋法(Breadth-first Search)，以Queue來實作，但此題要求Bottom-up，因此每算出一層都要把結果放在list的最前頭，這樣順序才會是對的

My C# Solution

/**
 * 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 queue = new Queue<TreeNode>();
        queue.Enqueue(root);
        while(queue.Count > 0)
        {
            var list = new List<int>();
            var currentLevelNumber = queue.Count;
            for (var i = 0; i < currentLevelNumber; i++)
            {
                var node = queue.Dequeue();
                list.Add(node.val);
                if (node.left != null) queue.Enqueue(node.left);
                if (node.right != null) queue.Enqueue(node.right);
            }
            result.Insert(0, list);
        }
        return result;
    }
}