107. Binary Tree Level Order Tra

递归真的不怎么会写。。

Java，这里使用了递归，借助另一个函数计算层级

/**
 * 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>> result=new LinkedList<List<Integer>>();
        count(result,root,0);
        return result;
    }
    
    public void count(List<List<Integer>> list,TreeNode root,int level)
    {
        if(root==null) return;
        if(level>=list.size())
        {
            list.add(0,new LinkedList<Integer>());
        }
        count(list,root.left,level+1);
        count(list,root.right,level+1);
        list.get(list.size()-level-1).add(root.val);
    }
}

Java，基于这题是求每行的节点值组成数组，用广搜更合适

public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
    List<TreeNode> toUse = new ArrayList<>();
    List<List<Integer>> retValue = new ArrayList<>();
    if( root != null)
    {
        toUse.add(root);
    }
    
    while (toUse.size() != 0)
    {
        List<Integer> result  = new ArrayList<>();
        List<TreeNode> next = new ArrayList<>();
        for (TreeNode node : toUse)
        {
            result.add(node.val);
            
            if (node.left != null)
            {
                next.add(node.left);
            }
            
            if (node.right != null)
            {
                next.add(node.right);
            }
        }
        retValue.add(result);
        toUse = next;            
    }
    Collections.reverse(retValue);
    return retValue;
}