314. Binary Tree Vertical Order Traversal

Posted 2020-08-09 咖啡中不塌缩的方糖

Given a binary tree, return the vertical order traversal of its nodes‘ values. (ie, from top to bottom, column by column).

If two nodes are in the same row and column, the order should be from left to right.

Examples:

 

1. Given binary tree [3,9,20,null,null,15,7],
   

      3
     / /   9  20
       /   /    15   7
   

    

   return its vertical order traversal as:

   [
     [9],
     [3,15],
     [20],
     [7]
   ]
   

2. Given binary tree [3,9,8,4,0,1,7],
   

        3
       /   /     9   8
     /\  / /  \/   4  01   7
   

    

   return its vertical order traversal as:

   [
     [4],
     [9],
     [3,0,1],
     [8],
     [7]
   ]
   

3. Given binary tree [3,9,8,4,0,1,7,null,null,null,2,5] (0‘s right child is 2 and 1‘s left child is 5),
   

        3
       /   /     9   8
     /\  / /  \/   4  01   7
       /   /     5   2
   

    

   return its vertical order traversal as:

   [
     [4],
     [9,5],
     [3,0,1],
     [8,2],
     [7]
   ]
   

 

 

广度优先的题，遍历整个tree

 public IList<IList<int>> VerticalOrder(TreeNode root) {
        var res = new List<IList<int>>();
        if(root == null) return res;
        var hashtable = new Dictionary<int,List<int>>();
        
        Queue<TreeNode> queue = new Queue<TreeNode>();
        Queue<int> queueDeep = new Queue<int>();
        queue.Enqueue(root);
        queueDeep.Enqueue(0);
        while(queue.Count >0 )
        {
            var a = queue.Dequeue();
            int deep = queueDeep.Dequeue(); 
            if(a.left != null)
            {
                queue.Enqueue(a.left);
                queueDeep.Enqueue(deep+1);
            }
            
            if(a.right != null)
            {
                 queue.Enqueue(a.right);
                 queueDeep.Enqueue(deep-1);
            }
            
            if(hashtable.ContainsKey(deep))
            {
            var temp = (List<int>)hashtable[deep];
            temp.Add(a.val);
            hashtable[deep] = temp;
            }
            else
            {
            hashtable.Add(deep,new List<int>{a.val});
            }
        }
        var sortHashtable = hashtable.OrderByDescending(x=>x.Key).ToDictionary(x=> x.Key,x=>x.Value);
         foreach(var pair in sortHashtable)
         {
             res.Add(pair.Value);
         }
         return res;
    }