算法练习之二叉树的最大深度，二叉树的层次遍历 II

1.二叉树的最大深度

给定一个二叉树，找出其最大深度。

二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。

说明: 叶子节点是指没有子节点的节点。

示例：
给定二叉树 [3,9,20,null,null,15,7]，

    3
   / 
  9  20
    /  
   15   7

返回它的最大深度 3 。

java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        if(root==null) return 0;
        int left = maxDepth(root.left)+1;
        int right = maxDepth(root.right)+1;
        int max = left>right?left:right;
        return max;
    }
}

php

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     public $val = null;
 *     public $left = null;
 *     public $right = null;
 *     function __construct($value) { $this->val = $value; }
 * }
 */
class Solution {

    /**
     * @param TreeNode $root
     * @return Integer
     */
    function maxDepth($root) {
        if($root == NULL) return 0;
        $left = $this->maxDepth($root->left)+1;
        $right = $this->maxDepth($root->right)+1;
        $max = max($left,$right);
        return $max;
    }
}

简化

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     public $val = null;
 *     public $left = null;
 *     public $right = null;
 *     function __construct($value) { $this->val = $value; }
 * }
 */
class Solution {

    /**
     * @param TreeNode $root
     * @return Integer
     */
    function maxDepth($root) {
        return $root == NULL ? 0 : max($this->maxDepth($root->left), $this->maxDepth($root->right)) + 1;
    }
}

2.二叉树的层次遍历 II

给定一个二叉树，返回其节点值自底向上的层次遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）

例如：
给定二叉树 [3,9,20,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7
返回其自底向上的层次遍历为：

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

java

class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {

        List<List<Integer>> lists = new ArrayList<>();
        if(root==null) new ArrayList<>();
        func(lists,root,0);
        for(int i=0,j=lists.size()-1;i<j;i++,j--){
            List<Integer> tmp = lists.get(i);
            lists.set(i,lists.get(j));
            lists.set(j,tmp);
        }
        return lists;
    }

    private void func(List<List<Integer>> lists,TreeNode root,int level){

        if(root==null) return;
        if(lists.size()==level){
            List<Integer> tmp = new ArrayList<>();
            tmp.add(root.val);
            lists.add(tmp);
        }else{
            lists.get(level).add(root.val);
        }
        func(lists,root.left,level+1);
        func(lists,root.right,level+1);
    }
}

TreeNode 

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

测试

import java.util.List;

public class Main {
    public static void main(String[] args) {
        Solution ss = new Solution();
        TreeNode root = new TreeNode(3);

        TreeNode a1 = new TreeNode(9);
        TreeNode a2 = new TreeNode(20);
        root.left = a1;
        root.right = a2;

        TreeNode b1 = new TreeNode(15);
        TreeNode b2 = new TreeNode(7);
        a2.left = b1;
        a2.right = b2;
        List<List<Integer>> lists = ss.levelOrderBottom(root);
        System.out.print("result----"+ lists.toString());
    }
}

 php

class TreeNode{
    public $val   = null;
    public $left  = null;
    public $right = null;
    public function __construct($value)
    {$this->val = $value;}
}

class Solution {

    /**
     * @param TreeNode $root
     * @return Integer[][]
     */
    public function levelOrderBottom($root)
    {
        if (empty($root)) {
            return [];
        }
        $lists = [];
        $lists = $this->func($lists, $root, 0);
        for ($i = 0, $j = count($lists) - 1; $i < $j; $i++) {
            $tmp      = $lists[$i];
            $lists[$i] = $lists[$j];
            $lists[$j] = $tmp;
            $j--;
        }
        return $lists;
    }

    public function func(&$lists, $root, $level = 0)
    {
        if (empty($root)) {
            return null;
        }

        if (count($lists) == $level) {
            $lists[$level][]=$root->val;
        } else {
            $lists[$level][]= $root->val;
        }
        $this->func($lists, $root->left, $level + 1);
        $this->func($lists, $root->right, $level + 1);
        return $lists;
    }
}

测试

//[3,9,20,null,null,15,7]
$root = new TreeNode(3);

$a1   = new TreeNode(9);
$a2   = new TreeNode(20);
$b1   = new TreeNode(15);
$b2   = new TreeNode(7);

$root->left  = $a1;
$root->right = $a2;

$a2->left  = $b1;
$a2->right = $b2;

$aa = new Solution();
$rs = $aa->levelOrderBottom($root);
var_dump(json_encode($rs));