102. 107.Binary Tree Level Order Traversal

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Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3   / \  9  20    /  \   15   7

return its level order traversal as:

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

分析: 使用层次遍历,递归时设置参数le,表明当前结点属于第几层,然后将其加入第几个list中既可;

<span style="font-size:14px;"><span style="font-size:14px;">/** * 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>> levelOrder(TreeNode root) {        List<List<Integer>> ans=new ArrayList<List<Integer>>();        if(root==null)            return ans;        level(root,1,ans);        return ans;    }        public void level(TreeNode root,int le,List<List<Integer>> ans){        if(root==null)            return;        if(ans.size()<le){            List<Integer> list=new ArrayList<Integer>();            list.add(root.val);            ans.add(list);                    }        else            ans.get(le-1).add(root.val);                level(root.left,le+1,ans);        level(root.right,le+1,ans);    }}</span></span>

107. Binary Tree Level Order Traversal II

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]]

思路：在上题基础上，设置一个全局变量max，用于记录当前搜索最深深度，然后属于level层的节点应该存放在max-level个list中。

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    int max=0;    public List<List<Integer>> levelOrderBottom(TreeNode root) {        List<List<Integer>> ans=new ArrayList<List<Integer>>();        levelHelp(ans,root,1);        return ans;    }        public void levelHelp(List<List<Integer>> ans,TreeNode root,int level){        if(root==null)            return;        if(level>ans.size()){            List<Integer> list=new ArrayList<Integer>();            list.add(root.val);            ans.add(0,list);            max=ans.size();                   }else{            ans.get(max-level).add(root.val);        }                levelHelp(ans,root.left,level+1);        levelHelp(ans,root.right,level+1);    }}

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