[LeetCode]Binary Tree Level Order Traversal II(JAVA)

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这道题我真的是使出了洪荒之力啊

/** * 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) {        TreeNode p = root;        List <List<Integer>> allNum = new LinkedList<List<Integer>>();        Map<TreeNode,Integer> nodeMap = new LinkedHashMap<TreeNode,Integer>();        List<TreeNode> nodeList = new LinkedList<TreeNode>();        if(root ==null)            return allNum;        nodeList.add(root);        nodeMap.put(root,0);        //Iterator it = nodeList.iterator();        int level = 0,preLevel = -1,i = 0;        while(i<nodeList.size()){            p = (TreeNode)nodeList.get(i++);            level = nodeMap.get(p);            if(p.left !=null){                nodeMap.put(p.left,level+1);                nodeList.add(p.left);                //System.out.println(level);            }            if(p.right != null){                nodeMap.put(p.right,level+1);                nodeList.add(p.right);            }        }        Iterator it = nodeList.iterator();        //it = nodeList.iterator();        //level = 0;        while(it.hasNext()){            p = (TreeNode)it.next();            try{            level = nodeMap.get((TreeNode)p);                }            catch (Exception e)            {                e.printStackTrace ();            }            if(preLevel == level)                (allNum.get(0)).add(p.val);            else{                List nList = new LinkedList<Integer>();                nList.add(p.val);                allNum.add(0,nList);                preLevel = level;            }        }        return allNum;    }}

在来一种递归的方法

public class Solution {    public List<List<Integer>> levelOrderBottom(TreeNode root) {        List<List<Integer>> result = new ArrayList<>();traversalTree(root, 0, result);;Collections.reverse(result);return result;}private void traversalTree(TreeNode node, int level, List<List<Integer>> list){if(node == null){return;}if(list.size() <= level){list.add(new ArrayList<Integer>());}list.get(level).add(node.val);traversalTree(node.left, level+1, list);traversalTree(node.right, level+1, list);    }}

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