【LeetCode】107.Binary Tree Level Order Traversal II

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题目：

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,#,#,15,7},

    3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

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

解答一：

使用队列进行水平遍历，每行结果放到栈里，再弹出到List里。

代码：

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */import java.util.*;public class Solution {    public class NewNode {TreeNode node;int level;NewNode(TreeNode m, int l) {node = m;level = l;}}public List<List<Integer>> levelOrderBottom(TreeNode root) {if (root == null)return new Vector<List<Integer>>();Queue<NewNode> nodeQueue = new LinkedList<NewNode>();Vector<Integer> cur = new Vector<Integer>();Stack<Vector<Integer>> retStack = new Stack<Vector<Integer>>();int lastLevel = 0;nodeQueue.offer(new NewNode(root,0));while (!nodeQueue.isEmpty()) {NewNode head = nodeQueue.poll();if (head.level > lastLevel) {retStack.push(cur);cur = new Vector<Integer>();lastLevel = head.level;}cur.add(head.node.val);if (head.node.left != null) {nodeQueue.offer(new NewNode(head.node.left, lastLevel + 1));}if (head.node.right != null) {nodeQueue.offer(new NewNode(head.node.right, lastLevel + 1));}}retStack.push(cur);List<List<Integer>> ret = new Vector<List<Integer>>();while (!retStack.isEmpty()) {ret.add(retStack.pop());}return ret;}}

解答二：

用递归进行水平遍历，结果保存到List<List<Integer>>对象里，再逆序输出到返回值里

代码：

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */import java.util.*;public class Solution {   public List<List<Integer> > result = new Vector<List<Integer> >();public void levelTraversal(TreeNode root,int level){if(root == null)return;if(level == result.size()){result.add(new Vector<Integer>());}result.get(level).add(root.val);levelTraversal(root.left,level+1);levelTraversal(root.right,level+1);}public List<List<Integer>> levelOrderBottom(TreeNode root){levelTraversal(root,0);List<List<Integer>> ret = new Vector<List<Integer>>();for(int i=result.size();i>0;i--){ret.add(result.get(i-1));}return ret;}}

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